84 C10 Brakes

[Henry]

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 

[Ronno6]

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.........

[Ronno6]

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...

[Henry]

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

[Ronno6]

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.

[Henry]

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

[Ronno6]

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...........

[bd]

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.

[Ronno6]

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.

[bd]

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.

[Ronno6]

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........

[Henry]

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.

[Ronno6]

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 ?

[Henry]

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

[Ronno6]

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.