84 C10 Brakes

[Henry]

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

[Henry]

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

[Ronno6]

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!

[Ronno6]

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..........

[Ronno6]

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.

[bd]

How long before you replace the combination valve, again?  What grade lining did you install on the rear axle?

[Henry]

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

[Ronno6]

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.

[Ronno6]

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.

[Ronno6]

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.

[Henry]

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

[Ronno6]

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.............

[Ronno6]

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.

[Henry]

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

[Ronno6]

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....