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73-87 Chevy _ GMC Trucks => Brakes, Frame, Steering & Suspension => Brakes and Braking Systems => Topic started by: Ronno6 on June 10, 2018, 09:25:02 AM

Title: 84 C10 Brakes
Post by: Ronno6 on June 10, 2018, 09:25:02 AM
I have replaced the rear wheel cylinders, calipers and master cylinder.
Bled master cylinder on the bench and on the truck.
I have done 1 man and 2 man bleeds, gone thru 2 quarts of fluid.
No air visible in the bleeder tubing.
No hoses,lines or joints are leaking.
Brakes are hard as rocks without vacuum boost, but, when engine is
running, the pedal goes way down, far past the stopping point when engine is off.
The truck will stop, but I am not confident that things are as they should be.
Sounds to me like my new (not rebuilt) master cylinder is defective.

What say y'all??
Title: Re: 84 C10 Brakes
Post by: bd on June 10, 2018, 10:54:27 AM
Power brakes feel softer than manual brakes.  It's the nature of the design.  Recheck the rear brake adjustment.  Make sure you adjust the shoes with the park brake cable adjustment backed off; that is, always adjust the shoes before adjusting the parking brake cable. 

If adjustment access for the rear brakes is through a window in the drum, remove the rear wheels, flip three lug nuts per side so the taper is pointing outwards and snug the drums to the axle flanges.  Adjusting one side at a time, tighten the star adjuster for each rear brake until you cannot rotate the drum by hand.  Then, using a small hook through the window to pull the adjuster away from the star, back off 12-15 clicks (teeth), or until you can rotate the drums with very little brake drag.  After the shoes are adjusted, readjust the park brake cable.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 11, 2018, 03:27:03 PM
I followed bd's instructions and there is no change.

The pedal is hard as a rock with engine off, and seems to be stiff when I apply more pressure with engine running.
When I ease off the pressure, the pedal travels downward, but stops before hitting the floor.
I believe this indicates the fluid bypassing an internal seal in the MC.
Title: Re: 84 C10 Brakes
Post by: bd on June 11, 2018, 03:40:29 PM
Try depressing the brake pedal lightly with the engine running.  Does the pedal gradually drop with light constant pressure?
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 12, 2018, 07:17:37 AM
couple questions, why did you replace all those parts? and thats about it for now ;)
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 07:21:39 AM
Try depressing the brake pedal lightly with the engine running.  Does the pedal gradually drop with light constant pressure?

Yup.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 07:23:59 AM
couple questions, why did you replace all those parts? and thats about it for now ;)

Rear cylinders were leaking.
Calipers had the bleeder valves frozen.
Master cylinder acted a bunch like the new one....pedal went way down and would not pump up.

What is the other question??
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 12, 2018, 08:28:07 AM
other question/s will follow. did you replace any hoses?
does the pedal first feel firm (like it should) then once you go past (lets say) 60% it goes straight to the floor or does it pretty much go straight to the floor?
Title: Re: 84 C10 Brakes
Post by: bd on June 12, 2018, 08:53:23 AM
Try depressing the brake pedal lightly with the engine running.  Does the pedal gradually drop with light constant pressure?

Yup.

Recheck the entire system for fluid leaks, including close inspection of every inch of the hard and soft lines and their connections.  If there are no leaks and the brakes are properly adjusted and bled then your original posit regarding a bypassing M/C may be correct.
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 12, 2018, 09:51:37 AM
bd do you think the old MC and new MC could have the same problem? i mean i know stuff can be bad right out of the box but couldnt the problem lay in the brake lines/hoses
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 10:11:41 AM
bd do you think the old MC and new MC could have the same problem? i mean i know stuff can be bad right out of the box but couldnt the problem lay in the brake lines/hoses

That thought had occurred to me.
The MC was acting in a similar manner to the one on my F150.
I replaced it and all is fine.
As the C10 is 34 years old and had been sitting for at least 4 years, I figured it to be time.
If the new MC is good, then the old one probably was as well.
But if one is bad.................

I have not, as of this juncture, replaced any hoses or lines.
There are no visible leaks anywhere.

The snap ring on the original MC has the ears broken off.
Dunno if that means that someone had attempted to disassemble and it went wrong.............

At any rate, I'm gonna do a complete re-inspection and check parking brake adjustment.
I'll see how it is then.
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 12, 2018, 11:18:19 AM
one thing that im thinking is a hose is blistering causing the fluid to go to a hose bubble and not the the wheel cylinder or whatever
Title: Re: 84 C10 Brakes
Post by: bd on June 12, 2018, 11:24:12 AM
bd do you think the old MC and new MC could have the same problem? i mean i know stuff can be bad right out of the box but couldnt the problem lay in the brake lines/hoses

I'm not there to make firsthand observations, so must draw from experience with the posted symptoms balanced against how the system functions while doing the best I can to filter out assumptions that have been made.  Based on what has been posted, I suspect an adjustment issue and/or residual air trapped in the system or a simple lack of familiarity with the "feel" of GM power brakes to be the most likely cause(s).  Don't make the mistake of believing that any particular symptom has only one cause.  The original symptom may have been repaired only to be replaced by a similar substitute having a different cause.  Hence, all we can do at a distance is make suggestions for checks to be performed and evaluated in the hope that it is sufficient for the OP to discover the truth and resolve the issue.  Do you disagree?

There are two wrong decisions to make: one is to do nothing, the other is to "overthink" the symptoms rather than just looking.  Filter out that which you can definitively through observation and what remains is probably the cause.

...At any rate, I'm gonna do a complete re-inspection and check parking brake adjustment.
I'll see how it is then.

I think this ^^^^ is a good decision.

one thing that im thinking is a hose is blistering causing the fluid to go to a hose bubble and not the the wheel cylinder or whatever

Checking the hoses while an assistant steps on the brake pedal is part of a basic inspection that may have been overlooked initially.  If so, it should be done now.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 11:31:10 AM
Checking the hoses while the petal is applies id a good suggestion.
I have noticed that the rear brakes will lock up when sufficient pedal force is applied.
I had not mentioned this earlier because until I replaced the shoes and drums, the shoes and drums had been contaminates with brake fluid or grease, which will cause the drum brakes to grab.
So, new drums and shoes, adjusted (parking brake as well) and rear braking is balanced side to side,
but still will lock up earlier than the discs in front.

Possibly a hose or hoses have some give which they should not.
As the pedal is rock solid without engine vacuum, maybe manual only force is
insufficient to cause a hose to swell, but vacuum assist may provide the force to do so.

On to this step.......
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 11:36:14 AM
Checking the hoses while the petal is applies id a good suggestion.
I have noticed that the rear brakes will lock up when sufficient pedal force is applied.
I had not mentioned this earlier because until I replaced the shoes and drums, the shoes and drums had been contaminates with brake fluid or grease, which will cause the drum brakes to grab.
So, new drums and shoes, adjusted (parking brake as well) and rear braking is balanced side to side,
but still will lock up earlier than the discs in front.
And the pedal still goes rather far down.
With regard to m F150 brakes, when I applied them, I could feel braking occur, then subside.
I do not experience this with the Chevy.

Possibly a hose or hoses have some give which they should not.
As the pedal is rock solid without engine vacuum, maybe manual only force is
insufficient to cause a hose to swell, but vacuum assist may provide the force to do so.

On to this step.......
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 12, 2018, 11:45:18 AM
bd do you think the old MC and new MC could have the same problem? i mean i know stuff can be bad right out of the box but couldnt the problem lay in the brake lines/hoses

I'm not there to make firsthand observations, so must draw from experience with the posted symptoms balanced against how the system functions while doing the best I can to filter out assumptions that have been made.  Based on what has been posted, I suspect an adjustment issue and/or residual air trapped in the system or a simple lack of familiarity with the "feel" of GM power brakes to be the most likely cause(s).  Don't make the mistake of believing that any particular symptom has only one cause.  The original symptom may have been repaired only to be replaced by a similar substitute having a different cause.  Hence, all we can do at a distance is make suggestions for checks to be performed and evaluated in the hope that it is sufficient for the OP to discover the truth and resolve the issue.  Do you disagree?

There are two wrong decisions to make: one is to do nothing, the other is to "overthink" the symptoms rather than just looking.  Filter out that which you can definitively through observation and what remains is probably the cause.

i 100% agree, just was throwing out that idea about the hose and wasnt sure if it was a plausible idea or not.



Checking the hoses while the petal is applies id a good suggestion.
I have noticed that the rear brakes will lock up when sufficient pedal force is applied.
I had not mentioned this earlier because until I replaced the shoes and drums, the shoes and drums had been contaminates with brake fluid or grease, which will cause the drum brakes to grab.
So, new drums and shoes, adjusted (parking brake as well) and rear braking is balanced side to side,
but still will lock up earlier than the discs in front.

Possibly a hose or hoses have some give which they should not.
As the pedal is rock solid without engine vacuum, maybe manual only force is
insufficient to cause a hose to swell, but vacuum assist may provide the force to do so.

On to this step.......

does it pull to one side or the other?
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 11:59:43 AM
does it pull to one side or the other?
[/quote]

No. It stops pretty straight.
But, even if one front hose is weak, wouldn't the equalizing nature of discs prevent one working harder than the other?

And then, there is the proportioning valve..........
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 12, 2018, 02:13:29 PM
it might reduce the line pressure and it might stop straight but im not sure, never dealt with this issue. im almost on the line of "thats a gm brake system for you" but want to eliminate anything we can. much like my sig says
"When you have eliminated the impossible, whatever remains, however improbable, must be the truth" ~Sherlock Holmes
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 12, 2018, 03:48:48 PM
it might reduce the line pressure and it might stop straight but im not sure, never dealt with this issue. im almost on the line of "thats a gm brake system for you" but want to eliminate anything we can. much like my sig says
"When you have eliminated the impossible, whatever remains, however improbable, must be the truth" ~Sherlock Holmes

It is not usually a good idea to have the rears lock up but not the front....especially on wet roads.
If there is a weak spot in a hose, that would reduce pressure to the calipers and gobble up pedal travel.
Kinda like an aneurysm in the brake system...........

Anyway, as I said, the truck had been sitting for at least 4 years. The MC reservoir was empty, but there was some
fluid still in the lines. Dunno if the proportioning valve went without fluid or not.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 07:29:19 AM
When I replaced the calipers, I thought that I had the 1" thick discs for the JB3 system.
I based this on the VIN, as I  do not have the RPO sticker.
I was wrong....I have 1.25" thick rotors. JB5?
However, the rear drums are 11" X 2", with 1" bore cylinders. JB3

These 2 do not go together (?)

Am I chasing my tail here?
Do I need to replace front system in order to get balanced braking?
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 26, 2018, 07:58:58 AM
jb/d 3 and 5 use the same master cylinder so i would assume those are comparable
Brake Tech, what do all the numbers mean.. (http://forum.73-87chevytrucks.com/smforum/index.php?topic=28592.0)
 
take a read here
Brake System and Diagnosis (http://forum.73-87chevytrucks.com/smforum/index.php?topic=33548.0)

Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 08:21:53 AM
Thanks.
I have previously read thru those.

I do not know if piston sizes differ between JB3 and JB5 calipers.
I do know that the rear cylinders differ in bore sizes" JB3: 1"; JB5 : 15/16"
Proportioning valve is the same for both systems.
The larger bore size in the rear cylinders would require more fluid to energize?

I have changed every hard part in the system.
Everything but hoses and hard lines.

Braking has improved, but pedal travel is still excessive in my estimation,
and rears still tend to lock up every now and then.
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 26, 2018, 08:33:39 AM
seams as if your very knowledgeable and know what youre doing but did you bleed your brakes  according to the "Brake system and diag."?
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 26, 2018, 08:38:22 AM
I didn't see any comments about the dash light.  If the system is off balance, would the proportioning valve switch not cause the light to come on?  And if it is tripped, it would prevent whatever system is an issue from being bled since the piston in the valve blocks the line between the m/c and that part of the system. 
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 08:43:11 AM
seams as if your very knowledgeable and know what youre doing but did you bleed your brakes  according to the "Brake system and diag."?

Yessiree.
I have bled the master cylinder, both on the bench and in the truck, and have bled each wheel , RR,LR,RF, then LF.
I have adjusted the parking brake.
The brakes all work, just not sure abut the balance.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 08:48:25 AM
I didn't see any comments about the dash light.  If the system is off balance, would the proportioning valve switch not cause the light to come on?  And if it is tripped, it would prevent whatever system is an issue from being bled since the piston in the valve blocks the line between the m/c and that part of the system.

No dash lite. Though I have not tested to see if bulb in dash is good.........
When bleeding, rear brakes don't seem to put out much volume, but, as they are working, I would have to think the volume is sufficient.
I am still not comfortable with pedal travel.
Without vac assist, the pedal is hard and travel stops higher up than with vac assist.
With engine running the pedal travels way down after first resistance is felt, then firms up near the bottom.
I am still thinking that there may be some internal bypassing going on, but I really don't notice any pedal fade.

When the MC in my F150 was bypassing, I could feel brakes start to engage then release with given pedal travel.
the new MC cured that problem.
Title: Re: 84 C10 Brakes
Post by: zieg85 on June 26, 2018, 08:54:12 AM
after reading and re-reading this post my thoughts are a bad "new MC"  What you describe about the power booster is how they work with no vacuum to it.
Title: Re: 84 C10 Brakes
Post by: Henry on June 26, 2018, 10:01:15 AM
Hi Ronno6:
Does your truck have the combo valve (aka proportioning valve)? Some of your symptoms indicate that this valve may be faulty or have residual air in it. My 76 has the combo valve mounted on the drivers side engine frame member down by the steering gear. My combo valve has a button on one end of it that must be held down while bleeding the system in order to make sure you get all the air out of the system.
Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 10:06:33 AM
I just replaced the prop valve.
The plastic bleeder tool accompanied it, but I did not use it, as I have not really seen a tutorial on how to do that....
(I'm not as smart as I appear to be....)

I just watched a short video on that.
Hope I didn't mess it up...........
I may need to recenter the valve.
But, as I said, the brake light is not illuminated, so hopefully all is good.....
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 26, 2018, 10:26:37 AM
the brake light should illuminate when you first turn the key "on" or if you ground out the wire that connects to the P valve
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 11:09:03 AM
the brake light should illuminate when you first turn the key "on" or if you ground out the wire that connects to the P valve

Yup. Nope...no brake lamp on the instrument cluster is lighting........
I wired the switch connector directly to the neg battery terminal....nuttin.
I removed the fan shroud again to gain access to the prop valve.
I unsctrewed the brake light switch and inserted the bleeder tool.
The tool threaded all the way in, so I conclude that the valve is still centered. Whew!
With the tool installed, I re-bled the brakes....again. (not redundant as I had already bled and re-bled the system.)
No change to the function of the system.

When depressing the pedal with engine running, it meets good resistance high up, but will
go way down rather slowly.
I am still thinking master cylinder....again.

I think I'll call this truck the "againmobile."
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 11:31:37 AM
I have seen posted on another forum that the JB5 calipers have a larger, 2.94: bore size, but have not been able to find out the bore size of the JB3 calipers.
The larger caliper bores may mean a delay in application.
But, the JB5 rear cylinders have a SMALLER bore than the JB3
One would think that the JB5 would activate sooner than the JB3 rears.

How do the 2 stages of the MC work?
Does the first stage send fluid to front and rear, or just one or the other??

I dunno. Maybe I need to back off on the parking brake some more............
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 26, 2018, 02:02:46 PM
Another new MC,
Another brake bleed (with the centering tool in the prop valve this time)
Another parking brake adjustment, and


NO CHANGE...........

About all that  is left is to revert the front brakes back to JB3 or upgrade rears to JB5.
Barring any other revelations.........

According to NAPA, the caliper piston bores are the same between JB3 and JB5 calipers.
I fail to see where that can cause an imbalance.
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 26, 2018, 11:48:07 PM
Which mc are you running?  Going back through this thread like 6 times. 'againmobile' . You said you changed the mc.  If the mc bore is too small, you'll get a soft pedal and grabby brakes. (you say it locks up sometime)  If the mc bore is too big, you'll get a hard pedal and less caliper pressure.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 27, 2018, 07:36:34 AM
Which mc are you running?  Going back through this thread like 6 times. 'againmobile' . You said you changed the mc.  If the mc bore is too small, you'll get a soft pedal and grabby brakes. (you say it locks up sometime)  If the mc bore is too big, you'll get a hard pedal and less caliper pressure.

From what I have found the master cylinder is the same for either JB3 or JB5
Some are listed as fitting "all US trucks with vacuum power brakes.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 27, 2018, 08:31:02 AM
I observed the 3 brake hoses in the system while my wife depressed the pedal w/motor running and could detect no lengthening or swelling in any of the hoses.

I am almost at a dead end here.
SOMETHING is going on here and I don;t know what.......

The only option I can try at this time is to switch rear cylinders to the smaller bore units for the JB5 system.
They will reduce the force applied to the shoes by 17%
Mavbe the lower volume will reduce pedal travel and increase pressure to the calipers.
That is an inexpensive swap, and, if that doesn't cure the problem, at lease I will have then already if I upgrade the rears to JB5.
I can still find no difference in bore size between JB3 and JB5 calipers.
Maybe the JB5 calipers have an internal pressure regulator in order to moderate the increased pressure from the double diaphragm brake booster ??

I am trying not to overthink this, but none of the simple solutions have worked.....
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 27, 2018, 10:39:31 AM
So, I tired AGAIN, this time blocking the pin open on the end of the prop valve, inserting the centering pin, and bleeding the front brakes....AGAIN.

And, AGAIN, no improvement.
I have ordered the 15/16" JB5 rear cylinders.
My last hope without major changes.
Title: Re: 84 C10 Brakes
Post by: bd on June 27, 2018, 12:33:35 PM
Have you verified that the rear shoes are properly sized to the drums?
Title: Re: 84 C10 Brakes
Post by: Henry on June 27, 2018, 12:48:57 PM
Hi Ronno6:
Another basic to check:
1. Recheck to make sure you did not install the combo valve backwards...on my truck it is possible to install it backwards...if this is possible on yours, the manufacturer should have made some "in" and "out" indications on the ports.

What is the status of your vacuum power brake unit?... has it been replaced or is it still the original one before the problems started? For my old truck, there were at least 4 different vacuum units available and I am wondering if maybe there is an incompatibility of your vacuum unit and the new master cylinder...or maybe the master cylinder push rod that goes between the two is not adjusted correctly, if it is adjustable. Some have an adjustable crown nut at the end of the rod. There are other things that can go wrong with the internal valving and seals of the vacuum unit to cause either hard pedal or soft pedal feel.

Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 27, 2018, 12:50:44 PM
Have you verified that the rear shoes are properly sized to the drums?

Yes, both are 11" dia x 2" wide
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 27, 2018, 12:53:04 PM
Hi Ronno6:
Another basic to check:
1. Recheck to make sure you did not install the combo valve backwards...on my truck it is possible to install it backwards...if this is possible on yours, the manufacturer should have made some "in" and "out" indications on the ports.

What is the status of your vacuum power brake unit?... has it been replaced or is it still the original one before the problems started? For my old truck, there were at least 4 different vacuum units available and I am wondering if maybe there is an incompatibility of your vacuum unit and the new master cylinder...or maybe the master cylinder push rod that goes between the two is not adjusted correctly, if it is adjustable. Some have an adjustable crown nut at the end of the rod. There are other things that can go wrong with the internal valving and seals of the vacuum unit to cause either hard pedal or soft pedal feel.

Regards,
Henry

Thanks for that.
The combo valve cannot be installed backwards.
The fittings are different sises.
Vacuum unit has not been changed. It works and holds vacuum after engine has shut off.
I did not see any adjustment capabilities.
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 27, 2018, 01:12:26 PM
curious but did you do any work to the brake shoes? have you verified everything is installed right and the primary shoes (front) is actually the smaller shoe?
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 27, 2018, 01:27:19 PM
curious but did you do any work to the brake shoes? have you verified everything is installed right and the primary shoes (front) is actually the smaller shoe?
Brake shoes are new.
Primary shoe (shorter lining) is installed to the front.
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 27, 2018, 10:05:30 PM
Brakes are hard as rocks without vacuum boost, but, when engine is
running, the pedal goes way down, far past the stopping point when engine is off.
Little to no vacuum = hard pedal.  Leak in the vacuum booster or faulty check valve can put air in the mc and cause a soft pedal.
Title: Re: 84 C10 Brakes
Post by: big_al273 on June 28, 2018, 12:09:38 AM

I just cannot understand why I cannot get my rear brakes to quit locking up under normal braking loads.


Are the adjusters in the drums over tightened?
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 28, 2018, 06:50:41 AM
Can you confirm, for sure, that you put the the brake lines on the right side of the reservoir when you replaced the m/c? If they're swapped, it will cause the rear to lock up.  The reservoirs are two sizes because of the different pressures needed for each system.

Also, the booster pushrod being misaligned will cause locking up since it preloads the m/c.  I can imagine it's possible to preload the rear more than the front since it's first in the m/c to the booster.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 07:21:43 AM

I just cannot understand why I cannot get my rear brakes to quit locking up under normal braking loads.


Are the adjusters in the drums over tightened?
I have tried both methods of initial adjustment: tightening til drum is locked, then backing off 12 notches, as well as adjusting so that there is no initial contact between drum and shoes.
Neither method has cured the problem.
Shoes,drums and wheel cylinders are all new.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 07:28:07 AM
Can you confirm, for sure, that you put the the brake lines on the right side of the reservoir when you replaced the m/c? If they're swapped, it will cause the rear to lock up.  The reservoirs are two sizes because of the different pressures needed for each system.

Also, the booster pushrod being misaligned will cause locking up since it preloads the m/c.  I can imagine it's possible to preload the rear more than the front since it's first in the m/c to the booster.

I put the lines on the new MC in the same orientation as when they came off the old one.
But, in fairness, as the problem existed with the old MC, it could be that they are reversed.
The lines coming off the MC are spiraled,tho, and I don't know if they can be swapped without really making a tangled mess.
I will examine to see if they are correct down to the prop valve.

For sake of clarification, which line should go to front and which to rear?

Front line is connected to rear brakes.

I have notice during bleeding that the front brakes get way more fluid than the rear brakes.
Is that proper?

I cannot think that the pushrod is misaligned.
The MC goes onto the booster without encountering any interference.
Good thought,tho........
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 07:31:27 AM
Brakes are hard as rocks without vacuum boost, but, when engine is
running, the pedal goes way down, far past the stopping point when engine is off.
Little to no vacuum = hard pedal.  Leak in the vacuum booster or faulty check valve can put air in the mc and cause a soft pedal.

The booster holds vacuum seemingly indefinitely after the engine is turned off.
Would that indicate that there is no leak?
Isn't the pushrod a solid rod all the way from pedal to MC ??
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 28, 2018, 07:39:46 AM
On any disc/drum setup, the big reservoir is for the front (disc) brakes.  Another thing to check is free play in the brake pedal.  There's should be 1/4" of free play, or it preloads the m/c.
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 28, 2018, 07:41:34 AM
I guess we have this conversation in two places.  I posted about the possibility of pedal play being an issue in the other string.  SHould be 1/4" or it preloads the m/c.  The rod must meet it's "at rest" position.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 07:41:53 AM
On any disc/drum setup, the big reservoir is for the front (disc) brakes.  Another thing to check is free play in the brake pedal.  There's should be 1/4" of free play, or it preloads the m/c.

I just reviewed the plumbing.
Front chamber connected to the rear brakes.

I will check pedal free play.

thanks...
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 07:43:35 AM
I guess we have this conversation in two places.  I posted about the possibility of pedal play being an issue in the other string.  SHould be 1/4" or it preloads the m/c.  The rod must meet it's "at rest" position.

Yes, though that was not my intent.
This thread was intended to deal with the system problems;
the other was to discuss the rear upgrade to JB5.
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 28, 2018, 10:27:54 AM
if the front is connected to the rear this will throw it all off since disk use 90 psi and drums use 40psi
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 11:06:23 AM
if the front is connected to the rear this will throw it all off since disk use 90 psi and drums use 40psi

The brake lines and fittings are different sizes.
The orifices in the MC are different alwo: Larger front for the 1/4" rear line, and smaller rear, for the 3/16" front line.
It is not possible to connect incorrectly......I just tried.
Unless someone has replaced the nuts with incorrect ones......
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 11:09:20 AM
Going to the smaller wheel cylinders did not correct the problem.
I guess the front brakes are just not getting enough  pressure.
They sure put out a lot of fluid when I bleed them.
I cannot imagine that the new prop valve is bad.
All that is left to consider is that I am using the JB5 rear cylinders w/the 2" brakes, and they are locking
up before the proportioning valve increases pressure to the discs.
Possibly the larger 2 3/4" shoes would provide that.
But, as of yet I have not found the backing plates to upgrade the rear system.

Or, maybe I should replace the hoses just because??

I really wish I knew if this problem was present with former owners.
SOMEBODY had to have changed the fronts to JB5.......
I dunno. 
I am at wits end.
Maybe time to admit defeat and take it in to the man..........
That's NOT why I bought this truck.....
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 28, 2018, 11:24:52 AM
you have the plastic master cylinder dont you? with the smaller chamber in the front? if so then yes rear is in the front
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 28, 2018, 11:28:24 AM
i was trying to figure out how the lines got switched up since theyre different sizes/nuts and are pretty much not bendable (too far from factory)
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 11:32:33 AM
you have the plastic master cylinder dont you? with the smaller chamber in the front? if so then yes rear is in the front

That is correct.
Most places specify the same part for either JB3 or JB5
Title: Re: 84 C10 Brakes
Post by: Henry on June 28, 2018, 11:48:08 AM
Hi Ronno6:

Manual bleeding technique verification:
1. Bleed starting with the valve closest to the mc and work your way to the furthest from the mc.
2. Combo valve held open during all bleeding.
3. Bleeder hose must remain submerged in a container of brake fluid while bleeding.
4. When bleeding, pump pedal up and pressure held before each opening of valve.
5. Open bleed valves only 3/4 turn and must be shut tightly before pedal reaches end of travel.

Concerning the vacuum power unit, there are various part numbers for the Chevy trucks but they all pretty much have the basic same design. The brake pedal pushrod inputs your braking desires and a separate master cylinder pushrod takes the command and applies it to the master cylinder with greater force. Between the two pushrods is air valving and springs which makes all the force multiplication to happen. Even if the unit holds vacuum, there still may be something going wrong inside. Rebuilding one of these things can be done but it requires some special tools and gauge or two. Maybe you can call around and see if there is a shop that specializes in the repair of these things and you can remove it and take it to them for testing just to verify it is OK. I also would encourage you to have someone verify that your new master cylinder is compatible with this older vacuum unit. Even before you do all this, a quick and inexpensive fix would be to replace the check valve on the outside of the unit and see if this makes a difference. At the firewall side of the vacuum unit, there should be a rubber boot covering the brake pushrod interface...just inside this boot should be an air filter...if you take the unit off you can inspect this filter.

You have done everything I would have done and everything is new except the vacuum unit...so that is why I suspect it...is your new mc exactly like your old mc?

Regards,
Henry

Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 12:15:23 PM
Hi Ronno6:

Manual bleeding technique verification:
1. Bleed starting with the valve closest to the mc and work your way to the furthest from the mc.
2. Combo valve held open during all bleeding.
3. Bleeder hose must remain submerged in a container of brake fluid while bleeding.
4. When bleeding, pump pedal up and pressure held before each opening of valve.
5. Open bleed valves only 3/4 turn and must be shut tightly before pedal reaches end of travel.

Concerning the vacuum power unit, there are various part numbers for the Chevy trucks but they all pretty much have the basic same design. The brake pedal pushrod inputs your braking desires and a separate master cylinder pushrod takes the command and applies it to the master cylinder with greater force. Between the two pushrods is air valving and springs which makes all the force multiplication to happen. Even if the unit holds vacuum, there still may be something going wrong inside. Rebuilding one of these things can be done but it requires some special tools and gauge or two. Maybe you can call around and see if there is a shop that specializes in the repair of these things and you can remove it and take it to them for testing just to verify it is OK. I also would encourage you to have someone verify that your new master cylinder is compatible with this older vacuum unit. Even before you do all this, a quick and inexpensive fix would be to replace the check valve on the outside of the unit and see if this makes a difference. At the firewall side of the vacuum unit, there should be a rubber boot covering the brake pushrod interface...just inside this boot should be an air filter...if you take the unit off you can inspect this filter.

You have done everything I would have done and everything is new except the vacuum unit...so that is why I suspect it...is your new mc exactly like your old mc?

Regards,
Henry

This is a lot to digest.
First, the bleeding technique.
The technique you cited seems to be exactly opposite every other procedure I have read:
start at the FURTHEST wheel and move progressively to the closest wheel.
In my case, that is" RR,LR,RF,LF.
I have tried 2 man bleeding, which is as you describe, and one man bleeding, using tubing
from the bleeder valve to into a jar with fluid in the bottom and the tubing in the fluid.
Both have yielded the same result.
(This procedure worked perfectly for my F150)
I have held the combo pin open and centering tool in place, although to be fair,
I did neither of those details in my initial bleeding procedures.
But, as the centering tool screws all the way in, I conclude that the valve is centered as it should be.

As for the booster, is there any failure scenario for the booster which would cause rear brakes to skid while fronts do not??
All the booster knows is pedal pressure in, boosted pressure out to the MC.
I do not believe it can differentiate between front and rear.
I may be wrong.....

Yes, both of the master cylinders which I have installed are identical to the original.
Title: Re: 84 C10 Brakes
Post by: irk_miller on June 28, 2018, 12:29:48 PM
The scenarios I pointed out in the other thread would give you exactly what you're dealing with.  The pushrod misalignment, no play (1/4" is what Chevy calls for) in the brake pedal which cause loading up on the brakes: or having the lines on the m/c reversed.  I also suggested a possible issue with the check valve, which Henry also suggests.  I really think you've covered everything else in the system, so the issue seems to point to the booster.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 12:59:15 PM
When I loosen the 2 nuts securing the MC to the booster, the MC does not move.
With the significant spring tension on the MC plunger, I would think that the  MC would move forward when the nuts were
loosened if the pushrod was misaligned.
There is 1/4" or better pre-engagement travel in the brake pedal.
With the MC nuts loosened, I step on the pedal and there is about that 1/4" of pedal travel before I observe the MC move.

I do not know how else to check for pushrod misalignment.

And, it is not possible to swap brake lines at the MC as thread sizes differ.

As for MC to booster compatibility, I have purchased MC's from 2 different sources. They are identical to each other, and the original.

I cannot find the inspection port/air filter in the vacuum unit Henry refers to.
I will see about replacing the check valve.

Thanks.
Title: Re: 84 C10 Brakes
Post by: bd on June 28, 2018, 01:24:48 PM
The technique ... I have read:
start at the FURTHEST wheel and move progressively to the closest wheel.
In my case, that is" RR,LR,RF,LF.

This procedure ^^^^^ is correct.  It is done this way to remove the greatest volume of contamination and air in the fewest number of strokes to re-establish clean fluid delivery to the cylinders.

All the booster does is transfer and amplify pedal pressure to the M/C.  It does not differentiate between the front versus the rear brakes.  Hydraulic pressure is administered by the M/C, moderated via the combination valve.  If either pushrod was too long the brakes wouldn't fully release, OR they might apply without driver pedal application as the fluid heats.  Neither symptom has been described.

IMSM, the front and rear brake M/C lines have different sized and threaded fittings to prevent inadvertent swapping of the lines.  It would take a concerted effort to mix them up.

It seems the real issue here is that the front brakes won't lock.  For the moment, concentrate your efforts on the front brakes.

Revisiting the basics, did you BENCH BLEED the M/C before installing it into the vehicle?
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 01:30:37 PM
The technique ... I have read:
start at the FURTHEST wheel and move progressively to the closest wheel.
In my case, that is" RR,LR,RF,LF.

This procedure ^^^^^ is correct.  It is done this way to remove the greatest volume of contamination and air in the fewest number of strokes to re-establish clean fluid delivery to the cylinders.

All the booster does is transfer and amplify pedal pressure to the M/C.  It does not differentiate between the front versus the rear brakes.  Hydraulic pressure is administered by the M/C, moderated via the combination valve.  If either pushrod was too long the brakes wouldn't fully release, OR they might apply without driver pedal application as the fluid heats.  Neither symptom has been described.

IMSM, the front and rear brake M/C lines have different sized and threaded fittings to prevent inadvertent swapping of the lines.  It would take a concerted effort to mix them up.

It seems the real issue here is that the front brakes won't lock.  For the moment, concentrate your efforts on the front brakes.

Revisiting the basics, did you BENCH BLEED the M/C before installing it into the vehicle?

I absolutely DID bench bleed the MC.
Serious volume of fluid comes out of the calipers when I bleed. I cannot believe there is air in the system anywhere.

One note:
When I had my wife depress the pedal with engine running and I was watching the hoses, I did note a slight movement of each caliper . I know they are getting pressure, just not enough??
Title: Re: 84 C10 Brakes
Post by: bd on June 28, 2018, 01:42:37 PM
This problem has been so persistent as to seem unique, though probably simple in nature.  Bear with me.  Did you try gravity bleeding of the brakes?
Title: Re: 84 C10 Brakes
Post by: bd on June 28, 2018, 01:44:15 PM
Please describe how you bled the M/C.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 02:07:57 PM
This problem has been so persistent as to seem unique, though probably simple in nature.  Bear with me.  Did you try gravity bleeding of the brakes?
I do not think so...I am not familiar with that method.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 02:13:47 PM
Please describe how you bled the M/C.
I clamped the MC body in a vise at the flat spots just ahead of the flange.
I removed the reservoir cap, and filled each res about 1/2 full.
I inserted tubing tapered nozzles into the discharge ports, with a piece of clear tubing
attached to each. The tubing looped over the top of the res and were held into the fluid
by a clip that attached to the center web between the 2 res sections.
I slowly depressed the plunger and allowed air to be pushed into the res, then slowly allowed the piston to return to
its at rest position. I repeated this til no more air came out of the ports.
Then, I removed the tubing nozzles and inserted plastic threaded plugs into the ports,
and installed onto the booster.
I even got my wife to help me further bleed on the truck.
Loosen 1 line fitting, have her depress and hold the pedal, then tighten the line fitting.
Repeat for both lines.
Title: Re: 84 C10 Brakes
Post by: Irish_Alley on June 28, 2018, 02:29:46 PM
when you put it on the truck and rebleed it did you get air?
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 02:41:46 PM
when you put it on the truck and rebleed it did you get air?

I cannot say for sure, as the fluid was squirting out around the fitting.

ps...replacing the booster check valve did not help.
Title: Re: 84 C10 Brakes
Post by: bd on June 28, 2018, 04:06:59 PM
Please describe how you bled the M/C.
I clamped the MC body in a vise at the flat spots just ahead of the flange.
I removed the reservoir cap, and filled each res about 1/2 full.
I inserted tubing tapered nozzles into the discharge ports, with a piece of clear tubing
attached to each. The tubing looped over the top of the res and were held into the fluid
by a clip that attached to the center web between the 2 res sections.
I slowly depressed the plunger and allowed air to be pushed into the res, then slowly allowed the piston to return to
its at rest position. I repeated this til no more air came out of the ports.
Then, I removed the tubing nozzles and inserted plastic threaded plugs into the ports,
and installed onto the booster.
I even got my wife to help me further bleed on the truck.
Loosen 1 line fitting, have her depress and hold the pedal, then tighten the line fitting.
Repeat for both lines.

^^^^ Perfect technique.

Gravity bleed:
Top off the M/C with fresh fluid.
Set the cover in place but do not snap down.
Open the LR bleed valve and let fluid drip into a collection pan for 20-30 minutes while maintaining the M/C fluid reservoir level above half-full.
Close the bleed valve and repeat the process at the RR, then RF, then LF, in turn.

Note that the LR is bled before the RR because the tee fitting in the rear axle brake line is generally mounted toward the right side of the pumpkin.  Otherwise, LR vs RR is arbitrary.

Sometimes, gravity bleeding will purge air that is reluctant to escape.

Is it possible for you to post a pic of either front caliper showing the hose and bleeder valve in relation to the "topside" of the caliper?
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 28, 2018, 05:20:21 PM
Thanks,
I'll give it a shot.
I should be able to snap a photo.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 29, 2018, 08:09:45 AM
An interesting observation:

The line fittings at the master cylinder are opposite sizewise to those at the compo valve.
The front. 1/4" line has the larger fitting at the mc, but the smaller at the pv.
Line size has been verified, and the fittings are definitely connected correctly.
Just seems odd...........

One other note.
I tested the pv switch function using a 12v test lamp connected to the + battery terminal.
The light did NOT illuminate when the pedal was pushed with vacuum assist.
Either the poppet does not move or the switch or valve is defective.
Next will be the same test with a bleeder screw opened.

I really do not understand what is going on here.
I clearly am not getting enough pressure to the calipers relative to the pressure going to the rears.
Maybe I will need to invest in a pressure kit:
https://www.amazon.com/KKmoon-Pressure-Tester-Braking-Testing/dp/B01DLW1O26/ref=pd_lpo_vtph_263_bs_tr_t_1?_encoding=UTF8&psc=1&refRID=8QQQRFP9207VHP7VQV03&dpID=514ZNcPPtPL&preST=_SY300_QL70_&dpSrc=detail
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 29, 2018, 10:46:52 AM
I performed the gravity bleed this morning.
Good news/bad news:
Good news is that pedal feel is greatly improved. Not nearly so mushy, and pedal stops much higher than before.
Progress...........
As with pedal pressure bleeding, not much fluid comes out the rear brakes, and a bunch comes out the calipers.
But, as it is the rears that are locking up, I don't see air in that end being a problem.

(http://)
Bad news is that the rear wheels still lock up.
Grrrrrrrrrrrrrrrrrr

Now. I have TRIED to get the prop valve switch to move, by opening a bleeder screw and pressing the pedal with ah 12v test light connected to + batt on one end and the probe connected to the prop valve switch terminal.
Zero,nil,nada. Tried front and rear and never got a light.

Looking like the NEW prop valve is bad??
Title: Re: 84 C10 Brakes
Post by: Henry on June 29, 2018, 12:58:43 PM
Hi Ronno6:
Well, BD is right about the conventional wisdom to bleed the furthest valve first and then progress to the closest valve. I gave you the opposite, as this is what was documented in my 1976 Chevy official service manual. Since every other reference book I have (Haynes, etc) says to start with the furthest valve, I guess this is right...maybe my service manual is a typo or the Chevy guys have changed their philosophy. Thinking back, I have always bled furthest first...I cannot recall bleeding closest first.

Concerning the air filter in the power vac unit, you can only get to the filter when the unit is removed...so dont bother checking it unless you remove the unit.

I see you recently mentioned your rear brakes locked up when your front ones did not...I did not notice being mentioned earlier. In the diagnosis section of my service manual it has a "REAR BRAKES SKIDDING PREMATURELY UNDER HARD BRAKE APPLICATION"...the Probable Cause is: Proportioning valve not controlling rear line pressure to prevent rear wheel lockup on hard brake applications....the Remedy is : Replace proportioning valve and bleed system. There is no power vac problem that could cause this symptom.

In the diagnosis section for the power vac unit, the symptoms (relevant ones to you) are:
1.NO BOOST-HARD PEDAL
2.SLOW BRAKE PEDAL RETURN
3.BRAKES GRABBY

Since your combo valve is new, it is hard to swallow that it is the problem...but I have seen situations (as I think BD would agree) that new replacement parts can be bad...you have mentioned that you tested it and cannot get it to give you a electical failure signal, right?

Regards,
Henry 

Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 29, 2018, 01:42:17 PM
You are correct.
I have tested 9 ways to Sunday and cannot get the prop valve switch to close.

After doing a gravity bleed as bd suggested, my pedal is much firmer now.
But the rears still lock up.
First mention of this was post #14, the next-to-last post on page #1.
It remains my biggest problem.

I have ordered a new prop valve.
 
More bleeding...again! (Brake fluid AND wallet!)
I just wanna put this issue to bed for good.........
Title: Re: 84 C10 Brakes
Post by: Ronno6 on June 30, 2018, 05:30:30 PM
Having made the discovery that my C10 already has the JB5 booster, I am beginning to think that maybe the entire differential may have been replaced at some time with one having JB3 rear brakes.
It appears that the rear brakes are the aberration in this situation.
The VIN calls for 4001-5000 GVW braking system,tho.

I remain befuddled...
Title: Re: 84 C10 Brakes
Post by: Henry on July 01, 2018, 01:41:55 PM
Hi Ronno6:
Is your C-10 a long or short bed?
What is the diameter of your front disks?
What is the first 7 characters of your VIN? (e.g. CCXXXXX)
On the ID plate on the door jamb, does it list a GVWR? GAWR FRONT? GAWR REAR?

The reason I ask these questions is to try and establish what the truck was most likely originally equipped with.

Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 01, 2018, 02:01:09 PM
Short bed Stepside
Front discs are the same diameter whether JB3 or JB5 @11.86" JB3 are 1" thick, JB5 are 1.25". Mine are 1.25"
first 7:  1GCCC14

The 4th character identifies it as 4001 to 5000# braking system (I believe that would indicate JB3)

The jamb sticker reads: GVWR 4980#
                                      GAWR FRT 2684#
                                      GAWR RR  2684#

Thanks.
Title: Re: 84 C10 Brakes
Post by: Henry on July 01, 2018, 11:40:39 PM
Hi Ronno6:
So I found the 84 GMC Light Truck service manual on the net and there is a chart that shows the braking systems available for 1984. When I compare this to my 76 manual, it appears that JB1 through JB8 is the same brake components from 1976 to 1984. I have attached your 1984 chart. I would have to agree with you in that your truck came from the factory as a JB5 and at some point someone swapped out the rear axle which had JB3 brake parts on it. I cant imagine going from 2.75 inch wide rear shoes (JB5) to 2.00 inch wide shoes (JB3) would be that significant of a change in the system...all else being equal. My 76 chart has a little more info in that it states that the C10s had JB1,3,5 options and the C20s had JB6,7 and the C30s had JB7, 8 options. Sorry I cant be of any help when it comes to there being a bunch of different wheel cylinder and caliper sizes that all fit...the manual does not discuss the fact that there are multiple sizes that are interchangeable.

Getting back to testing your combo valve for failure light operation, here is a copy of how the 1984 manual says to test it:

COMBINATION VALVE
Electrical Circuit Test
1. Disconnect wire from switch terminal and use a jumper
to connect wire to a good ground.
2. Turn ignition key on "ON" - warning lamp should
light. If lamp does not light, bulb is burned out or
electrical circuit is defective. Replace bulb or repair
electrical circuit as necessary.
3. When warning lamp lights, turn ignition switch off.
Disconnect jumper and reconnect wire to switch
terminal.
Brake Warning Light Switch Test
1. Raise vehicle on hoist. Attach a bleeder hose to a rear
brake bleed screw and immerse the other end of the
hose in a container partially filled with clean brake
fluid. Be sure master cylinder reservoir is full.
2. Turn ignition switch to "On"; open bleeder screw while
a helper applies moderate pressure to the brake pedal;
warning lamp should light. Close bleeder screw before
helper releases brake pedal. Reapply brake pedal with
moderate-to-heavy pressure; light should go out.
3. Attach the bleeder hose to a front brake bleeder screw
and repeat above test. Warning lamp action should be
the same as in Step No. 2. Turn ignition switch off.
4. If warning lamp does not light during Steps and 3 but
does light when a jumper is connected to ground, the
warning light switch portion of the combination valve is
defective.

I dont think this is how you stated you tested it...if you retest it to the above procedures and it fails it is time to get another combo valve. When you found those two part numbers of combo valves in the parts book, did it state what brake system they were for?....JB3 or JB5...?

Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 02, 2018, 07:16:01 AM
The VIN and GVW numbers indicate the JB3 option should have been installed from GM.
Hard to believe that someone would have swapped spindles and all the other stuff to go to JB5 on the front.
Lotsa expense and work.

As for testing, my brake light in the instrument panel does not work, so I just used a 12v test light.
I conducted the test as described, and could never get the light to illuminate.
Hence, I fear the CV is defective.

One interesting note: The JB3 brake components for the G10 van are identical to mine, except for the booster.

I have purchased a pair of backing plates, supposedly for the JB5 rear system.
As I am planning on a posi installation soon, I will upgrade to JB5 rear brakes at that time.
If these plates are not correct, I may consider rear disc conversion.

Of the 2 part #'s I found for the combo valves, the JB3 valve was superseded by one that I have found available.
I have not found the JB5 CV as of yet.

There are things about my C10 which lead me to believe that its  production was not normal.
The paint appears to have been changed during manufacture.
I see evidence that it started out as red, then repainted to red-orange.
There is red under the orange, and places on the dash where the red is visible.
The firewall is red-orange and has all the black tar sprayed in the normal areas.
Hard to imagine a body shop doing that.
Maybe the brake or axle installation chanted somewhere down the line............
I can't imagine a body shop doing that...........

Title: Re: 84 C10 Brakes
Post by: bd on July 02, 2018, 08:43:36 AM
Several times you've stated that the brake warning indicator on the dash "never" illuminates even when the brown wire to the differential pressure switch is directly grounded with the ignition switched on, yet you have never diagnosed why.  Relying on an inoperative indicator doesn't make sense and will derail your diagnosis.  Find out why the indicator doesn't function and repair it before proceeding with anything else.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 02, 2018, 09:27:39 AM
I conclude that the warning light lamp is burned out.
That is why I used a 12v test light............
IT never illuminates,either at least not when connected to the CV switch. (It lights up when touching a ground surface,tho.)
Is one not as valid as the other????

It is my conclusion that the test light does not illuminate when I test per the published procedure is that the CV is not operating correctly.

The reason I repeat it is because people keep asking.
Title: Re: 84 C10 Brakes
Post by: bd on July 02, 2018, 09:45:53 AM
Thanks for the clarification.  Substituting the test light, connected B+ to differential pressure switch pin, is valid.  Does the test light illuminate when connected between B+ and the combination valve body, verifying that the valve body is effectively grounded to frame and B-?

Don't forget to repair the dash indicator lamp at some point.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 02, 2018, 10:33:41 AM
Thanks for the clarification.  Substituting the test light, connected B+ to differential pressure switch pin, is valid.  Does the test light illuminate when connected between B+ and the combination valve body, verifying that the valve body is effectively grounded to frame and B-?

Don't forget to repair the dash indicator lamp at some point.

I tested and the CV brass body is grounded.

I have to conclude that the fest fails because the poppet in the CV does not move with pressure differential in either direction.

As for the dash light, I dissected the dash face and found that three IS no lamp for the brake warning light.
In fact, there is NO SOCKET !!
There IS a lens, however.
I will need to remove the cluster in order to investigate, and at present, I have not been successful in removing the speedo cable.(only a little skin...)

Sockets are the 5/8" bayonet units. Ordered some w/bulbs from good ol' ebay........
Title: Re: 84 C10 Brakes
Post by: Henry on July 03, 2018, 07:54:14 AM
Hi Ronno6:
Well, even if your new bulb and socket for the brake lamp switch does not fix your brake lamp circuit problem, it sounds as if your test lamp method of checking the CV is OK...hopefully the new CV will solve the rear lock up problem.

Without that RPO sheet, you probably will never really know what brake system the truck originally came with...these trucks have so many interchangable parts and the parts were so plentiful at one time that a lot of swapping occurred for economical reasons. For example, when I bought my 76 C-20, the RPO sheet said it came with a T400 transmission even though it had a T350 in it...worked perfectly fine. Now that I have seen your posting on the brakes, it made me look into my brakes...my truck has the JB7 brake configuration and my RPO sheet says I should have the J55 merchandizing option which is the dual diaphragm power unit...but I am pretty sure the unit I have on the truck is a single diaphragm unit...brakes work fine even when towing a 2000# load. I am thinking that the single vs dual diaphragm vac unit may not make a difference for your truck's problems...only if there is a internal problem with the unit you have.

Regards,
Henry
PS: I am not sure how the merchanizing options worked, but the J50 option was for single diaphragm vac unit and J55 was for dual vac unit. My 1976 service manual shows the C-10 stepside has the J50 option with JB3 and the J55 option with JB5.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 03, 2018, 01:35:27 PM
Is there ANY advantage to the truck or driver to balance the braking system by using the larger rear brakes
(which would complete the same JB5 system  as the fronts and the booster) vs balancing performance of the smaller
JB3 rear brakes by means of an adjustable proportioning valve??
Assuming, of course, not requiring the JB5 system due to higher loads............

There is a sizeable difference in cost, as I already have theJB3 rears, and the adjustable PV would be less than $40.00 vs the cost of the JB5 components.

Note: JB3 is listed as being for 4600 (GVWR?)
My truck is 4980 GVWR.
Should my brake system be JB5 ?
Title: Re: 84 C10 Brakes
Post by: Henry on July 04, 2018, 01:54:37 PM
Hi Ronno6:
I would say that there is an advantage to upgrading to rear JB5 vs just adding an adjustable pv:
1.Safety: your ID plate of GVWR lets someone know what they can safely haul...the braking system is part of the GVWR formulation...if you leave the JB3 rear brakes in, the true GVWR rating of your truck is less than the numbers (overall and rear) on your door jamb plate. If you keep the truck for yourself forever and dont loan it out to anyone there is no problem...but if you sell it or give it to someone else I would disclose the GVWR rating compromise because of the rear brakes.

2.Safety: If you add a adjustable proportioning valve, you must do some panic stop tests to make sure that the rear does not lock up to early. You have to do this testing on turns as well because if you lock up the rear on a panic turn braking, the rear could swap ends with the front from early lock up.

3. Safety: I personally dont like something adjustable hanging out there for anyone to touch after I have adjusted it for optimal safety. I have an aftermarket adustable prop valve on my car as a necessity to a brake upgrade, otherwise I would not have one.

For 1976, the C-10 stepside brake system splits for GVWR are:
GVWR 4900....JB1
GVWR 5300-5600....JB3
GVWR 5300-6200....JB5

Can you not find this info for 1984? In any case, I dont think the GVWR is a straightforward indication of what brake system your truck came with...I think that even if your frame and chassis was rated for lowest GVWR, they could have added a brake upgrade option within the options available listing.

Have you given up on that JB5 OEM CV? You know, I think you still need to establish what is the rear lock up problem before you start thinking on how optimize the brake system...even if you had the wrong OEM CV valve in there, and a mix of JB3 and JB5 parts, your rear would not lock up under normal braking conditions (non-hard stops), which is what you have described. Can you take that CV back to where you bought it and tell them it is defective and at least have them give you another one...so you can continue your troubleshooting?

Regards,
Henry

Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 04, 2018, 02:24:14 PM
This is the data for post 1980 Half tons:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=28592.0

I'm gonna call my brake system JB3.5........
Check out the G model (van) JB3 setup.................just lie mine!

I have found an AC Delco  JB5 CV for sale (superseded part #) and may give it a shot.

I do have another CV on the way from a different source.
We'll see what that does before I move on from there.

I have seen a JB5 rear brake system for sale, but for an older vehicle.
The drums, shoes, and cylinders are the same part #'s, but the backing plates are different.
The backing plates changes part #'s after 82, and went til at least 91.
I may have to give them a shot, but am afraid that something may be different like maybe the
overall depth could be different if the donor vehicle's axles were in or out farther than mine.
Gotta be a reason for part # changes from the factory.........

Henry,
Thanks for your continues contributions. Seems that some have lost interest.
Title: Re: 84 C10 Brakes
Post by: Henry on July 05, 2018, 12:13:21 PM
Hi Ronno6:
Check out this 1984 spec file from the GM Heritage Center:

https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Chevrolet-Trucks/1984-Chevrolet-Truck.pdf

It may or may not help you figure out what your truck originally came with...at least it will help you decide what you want your truck to become...it may also give you some clues as to what your paint originally was.

I would keep plugging away at just getting your brake system working properly (under low force braking conditions) and then later on start thinking about how you can get the rear axle ratio you want with upgraded brakes at the same time. For now, I honestly believe you can get your brakes bled with pedal pressure feeling normal with a mix-mash of brake components.

Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Henry on July 05, 2018, 12:16:47 PM
BTW, if you REALLY have to figure out what your truck originally came as, I think you can order a RPO sheet for your VIN from these guys at the Heritage Center...I havnt looked into it recently but I think they will find yours for a $50-$100 fee.
Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 05, 2018, 05:38:54 PM
Thanks, Henry.
I am aware of the Heritage Center and the documentation available from them.

As for the brakes:
First, I have made the seller of the combo valve aware that I am having trouble and he has been very interactive with me to troubleshoot. We'll see where that leads.
Second, I have purchased a complete right and left rear brake JB5 brake setup, minus drums.
If all else fails I will install that when I do the posi installation.

Pretty much all other projects are on hold with the C10 til I sort out the brakes and rear end,
with the exception of the instrument cluster rebuilding with working dash lights and warning lights.

Thanks for your interaction!
I appreciate it!
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 12, 2018, 07:47:37 AM
I understand the principles of weight transfer, and that the fronts do most of the stopping.
But, is of any consequence that the rears only lock up when traveling forward??
They do not lock up when in reverse..........
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 12, 2018, 01:04:01 PM
It was suggested to me by 2 totally separate people on the same day that I might have
a problem with the rear brake hose collapsing internally and acting like a check valve.
So, after much ado (good thing my wooden bed boards are in need of being replaced....)
I replaced the hose.
No change to the situation.

If replacing the CV again yields no results, I am stymied til I upgrade the rear brakes to JB5.
Title: Re: 84 C10 Brakes
Post by: bd on July 12, 2018, 01:38:50 PM
How long before you replace the combination valve, again?  What grade lining did you install on the rear axle?
Title: Re: 84 C10 Brakes
Post by: Henry on July 12, 2018, 01:48:15 PM
Hi Ronno6:
The reason your rears do not lock up when going in reverse is purely a weight transfer issue...all that weight from the front of the truck is being transferred to the rear brakes and so they cannot lock up as easily because of this extra weight force resisting the braking force that is applied to the rear brake circuit...even if the CV is allowing too much pressure to the rear circuit. My speculation is that your rears are locking up during forward travel braking because the rear is so light there is no weight opposing force to counteract a rear brake circuit that is applying too much pressure because the CV is not working right to modulate the rear circuit properly.

I bet all will be healed when you get a new CV...it would also be worthwhile to review the bleeding procedure they have in that 1984 manual and see if it differs any from what you do or have done.

Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 12, 2018, 02:14:02 PM
How long before you replace the combination valve, again?  What grade lining did you install on the rear axle?

I may do the valve and re-re-re-re-re-re-bleed in a day or 2.

The shoes are listed as premium, material: ceramic.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 12, 2018, 02:18:26 PM
Hi Ronno6:
The reason your rears do not lock up when going in reverse is purely a weight transfer issue...all that weight from the front of the truck is being transferred to the rear brakes and so they cannot lock up as easily because of this extra weight force resisting the braking force that is applied to the rear brake circuit...even if the CV is allowing too much pressure to the rear circuit. My speculation is that your rears are locking up during forward travel braking because the rear is so light there is no weight opposing force to counteract a rear brake circuit that is applying too much pressure because the CV is not working right to modulate the rear circuit properly.

I bet all will be healed when you get a new CV...it would also be worthwhile to review the bleeding procedure they have in that 1984 manual and see if it differs any from what you do or have done.

Regards,
Henry

I am inclined to agree...at least I  hope you are correct.
It is my understanding that CV units are not adjustable.
If the centering poppet is stuck on one side (F or R), there would be no braking on the other (?)

Fluid does pass thru to all 4 wheels, so, I wonder....

As for bleeding, I have found the gravity method to work best.
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 13, 2018, 09:10:31 AM
Don't forget to repair the dash indicator lamp at some point.
The brake warning light bulb and socket on the dash has been replaced.
It illuminates when the key is turned to start. not before, and goes out when key is released.

Title: Re: 84 C10 Brakes
Post by: Henry on July 13, 2018, 12:08:03 PM
Hi Ronno6:
Sounds as if your warning light circuit is working properly...on my 76, the light goes on when the ignition switch is in "START" (cranking) and goes out when switch is in the "ON" or "OFF" positions. If your warning light stays on when your ignition is "ON", then your CV warning switch is actuating. If everything is working normally the warning light on your dash should only come on only during cranking which is a indication that the bulb and circuit is functioning properly. Yes, the OEM CVs are not adjustable or serviceable.

Your newer truck may have a slightly different CV than my older truck but I think the basic internal operation is the same...what you call the "centering poppet" is what I think is the switch piston that is right under the switch terminal. If the piston gets pushed from one side or the other, it will not freely re-center itself because there are detents on the piston that must be overcome. It will only re-center itself if there is sufficient pressure on each side of the piston to re-center it by overcoming the detent force. Even if the piston is pushed to one side or the other, there is still passageways for the fluid to flow by the piston on both circuits to apply brake pressure assuming any pressure is available...in other words, the switch piston does not control fluid flow. I think earlier on this thread you had a sectional view of a CV that should show this. My 76 service manual says it requires 100 to 450 psi to overcome the detents. Once the system is bled and operating properly, a heavy application of the brake pedal while while parked should re-center the switch piston if it was offset for some reason.  Also, if your currently installed CV switch piston is pushed to one side, the warning light should be illuminated all the time when the ignition is "ON".
Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 13, 2018, 01:57:26 PM
Thanks, Henry.

I am beginning to question of my front calipers are getting all the pressure they need.
It takes a fair amount of pedal pressure to lock up the rear brakes, and the fronts should take over well before that in my opinion.
I don't know how to check this without purchasing a pressure gauge for the brake system.
I dunnio.............
Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 14, 2018, 10:18:31 AM
As I continue to mull over my balance problem, many things come to mind.
1. JB5 and JB3 utilize a common master cylinder.
2. Most accepted modern day wisdom is that disc/drum combination valves are universal.
3. JB3 and JB5 calipers share the same piston bore size.
4. JB5 rear brakes are 3/4" wider, yielding a 37.5% increase in friction surface (slightly more if you consider the 3/16" increase in diameter.)
5. JB3 rear wheel cylinder is 1" bore, while the JB5 is 15/16", a 14% or so reduction in applied force at a given pressure.
6. JB3 vacuum booster is single diaphragm; JB5 booster has a double diaphragm.
7. Front brake hose assemblies are different for JB5 than JB3.

The basic question is: can mixing these components result in a suitably performing system using only stock parts??

The JB5 rear brakes must normally operate under higher pressure, as a smaller bore cyl applying force to larger shoes would require a higher operating pressure.
With a universal combination valve, this would normally indicate that higher pressure would also be proportionately sent to the front calipers.
The increase in operating forces (given a common master cylinder) necessitate a more powerful booster.
If correct, this seems to make sense.

Now, let's mix components. JB5 Front, JB3 Rear.
When operator applied,the pressure builds more quickly due to JB5 booster causing the lower area rear shoes to exert more pressure against the narrower drums, resulting in premature rear wheel lock up.
As this is happening at a lower pressure than the wider brakes could handle without locking up, there is less pressure being sent via the combination valve to the front calipers.
It may also be possible that, as the system may be designed to operate under higher overall pressure, the JB5 calipers may have a smaller internal piston orifice in order to not be overly sensitive.
I have not been able to find any supporting pressure or caliper orifice data to support this.
It is all supposition on my part.
The perception is definitely the the fronts are not stopping adequately; they may not be receiving sufficient pressure before the rears lock up.
I have been in contact with a member on another forum who experienced exactly the same component mix situation as I am facing.
His F/R bias imbalance was only solved by upgrading the rears to JB5
There's gotta be a reason why.
But, if my analysis valid, I am pretty much spinning my wheels (fronts more so than rears ha,ha) until I install the JB5 rear system.
I received the components for that upgrade, and will hopefully get to that shortly.
Title: Re: 84 C10 Brakes
Post by: Henry on July 14, 2018, 11:57:11 AM
Hi Ronno6:
Your analysis of how the JB5 vs JB3 systems would work makes sense to me. Earlier I commented that the JB3 vs JB5 rear brakes should not make much difference with 3/4 inch width difference, but I should have done the math first before commenting...37.5%  surface area is significant to me especially on a short wheel base light rear end truck like yours.

I dont think the size of the vacuum booster plays into the equation though...dual diaphragm simply allows you to apply more pressure to any given master cylinder without you exerting more leg effort.

Since you have two problems (low front braking force and rear lockup) keep an open mind as to number of causes...you may have more than one cause such as a faulty CV and a malfunctioning vac booster. But I would still go the step by step process and replace the CV first and see what happens.

It is a little strange to me that the Chevy guys used as smaller rear wheel cylinder on the JB5 while a bigger one on the JB3...seems to me it should be bigger...maybe they did it after testing showed that the JB5 system behaved better with a smaller rear wheel cylinder...I bet the CV played into this decision.

Regards,
Henry

Title: Re: 84 C10 Brakes
Post by: Ronno6 on July 14, 2018, 02:07:50 PM
The DD booster provides more brake pressure (recquired by the JB5 system) without increased driver effort.

The smaller bore wheel cylinder of the JB5 does 2 things:
1.Smaller bore gives more travel per psi input from the MC.(quicker to energize the front calipers.)
2.Builds pressure more quickly to provide feedback to the CV to give more psi tho the front brakes.
The bias function of the CV is, I have been told, a static operation...no moving parts.
The increase pressure/fluid flow to the calipers is a function of the metering valve in the CV, and it opens in response to the back pressure from the rears. That is what is supposed to prevent rears from locking up.
By the time the rears in my hybrid system provide ample back pressure, the drums have already locked up.
An adjustable proportioning valve would serve to provide higher back pressure on the input side, but lower pressure to the rear cylinders.

I have been told that the stem under the rubber boot at the front brake section of the CV should show a little movement when the pedal is pressed. I will check mine to see if this occurs. If not, it may indicate that the metering section is not functioning, although I cannot believe that it could remain closed under the pressure from the MC.

I still feel that the JB5 calipers may be internally metered in order to prevent the system from being too sensitive.
Just my $0.02
One more possibility just occurred to me: as the front hoses are different part numbers for the JB3 and JB5, there may be a metering function to the JB5 assemblies. I can't understand why they would be any different from the JB3 hose assemblies....
Title: Re: 84 C10 Brakes
Post by: SUX2BU99 on August 05, 2018, 12:55:08 AM
This is a good discussion. Sorry to interrupt your thread but what visual difference does the JB3 vs JB5 master cylinder have? I have a hard pedal after taking my C10 out of winter storage and I know the front brakes are dragging but I don't feel it pull to one side so I don't think I have a collapsed brake hose. Just not sure how to identify the MC. My truck is somewhat of a mutt. It has the cast iron reservoir though with the single hold-down hoop. I have the bigger drums with wider 2.75" shoes but can't say they were stock or not. Thanks!
Title: Re: 84 C10 Brakes
Post by: Henry on August 05, 2018, 12:40:13 PM
Hi 99:
Sounds as if you have a 1984 JB5 system....the master cylinder may be the same one as the JB3...I do know that the 1984 JB5 had the dual diaphragm vac unit. The 1984 service manual may have pics of what the various master cylinders looked like. You also should page back on this thread or a related thread by Ronno and see that the moderators linked some images of various master cylinders...you should also go to the tech section of this forum to get more pics of master cylinders.
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on August 12, 2018, 09:06:23 PM
From what I have seen the cast iron reservoir master cylinder was used only in the JD3 and JD5 brake systems,
which were used in diesel powered trucks.
These were hydra boost systems, and I don't know how well this MC will work with Vacuum booster......
Title: Re: 84 C10 Brakes
Post by: Ronno6 on September 26, 2018, 08:57:51 AM
After all was said and done, the JB5 rear brake system is installed along woth 3.73 gears and a Yukon posi unit.

IT WORKS !!  NO MORE REAR WHEEL SKIDDING !!!!!

What a relief.............now.....on to the carb. That 1406 GOTTA GO........
Title: Re: 84 C10 Brakes
Post by: Henry on September 26, 2018, 12:26:33 PM
Hi Ronno6:
So after all is said and done on this issue, what do you attribute your rear lockup problem  and low front braking force to?

Regards,
Henry
Title: Re: 84 C10 Brakes
Post by: Ronno6 on September 26, 2018, 04:28:16 PM
Hi Ronno6:
So after all is said and done on this issue, what do you attribute your rear lockup problem  and low front braking force to?

Regards,
Henry

FWIW I believe that the JB5 rears are sent more pressure than the JB3 rears, due to the larger shoes but smaller bore cylinder.
This higher pressure would cause the narrower and smaller dia. JB3 shoes to lock up against the drums, especially as there was no load in the bed. This would happen prior to the combo valve sensing the pressure buildup and sending adequate pressure to the front calipers.
The system is now working as designed,

I believe that the mix of systems could not achieve balance.
My post of July 14th may explain the hows and whys, dunno 4 sure.
On another forum I ran across a member who had experienced the exact same symptom with the exact same mix of systems.
The only fix that worked for him was the one which worked for me.
Remote as it may be to believe, maybe the GM engineers had method to their madness........