C10 Unbalanced F/R

[Ronno6]

When I purchased my 84 C10, the rear brakes would always skid before stopping.
I replaced front cylinders,drums,shoes,rear cylinders, master cylinder,combination valve and hoses.
Virtually everything but hard lines and rotors.
Nothing helped.
While sourcing the components, I discovered that the rears were the standard JB3, while the fronts were heavier JB5. I couldn't imagine that to me the problem....
I had read a thread on another forum about an owner who had the exact same setup....and symptom.
He was only able to correct it by installing JB5 brakes on the rear.
(funny to note that the same setup WAS standard on vans...)
So,after much searching and $$$$$, I found the necessary parts for the JB5 rear brakes.(Backing plates are hard to come by...)
I installed the new JB5 system while I had the rear axle out to install the new chunk and gearing.
Perfect! No more rear skidding!
The engineering explanation is rather long and drawn out, but it works now.

 

[Blue Ox]

Thank you for sharing that. I've fought the brake battle myself and sometimes GM just doesn't give you the full story. Nice that you got it worked out.

 

[bucket]

Here's what has me confused, the JB3 and JB5 front brakes are the same other than the thickness of the rotor. In theory, clamping force should be the same, but one will just dissipate heat faster and is less likely to experience brake fade.

What am I missing?

 

[Ronno6]

Here's what has me confused, the JB3 and JB5 front brakes are the same other than the thickness of the rotor. In theory, clamping force should be the same, but one will just dissipate heat faster and is less likely to experience brake fade.

What am I missing?

Don't think that I didn't lay awake at night asking myself the very same question.........
The calipers are different, of course, to handle the thicker rotors. They must be metered differently internally from the JB3 calipers to reduce the flow of fluid. This would be necessary as the larger rear shoes require more fluid pressure to apply suitable braking force (wider shoes, larger diameter, and smaller bore brake cylinder.) The buildup of rear brake line pressure is what triggers the combination valve to begin sending pressure to the front system. The JB3 rear shoes would over pressurize and lock up the drums before sufficient rear brake line pressure built up in order to send pressure forward.
That's my best guess.
One cannot argue with results...........

 

[Ronno6]

Here's what has me confused, the JB3 and JB5 front brakes are the same other than the thickness of the rotor. In theory, clamping force should be the same, but one will just dissipate heat faster and is less likely to experience brake fade.

What am I missing?

ps...the rotors are the same diameter.
I can't remember, but pads may be the same.
That is where the similarity ends.
Rotors are different thickness,
Calipers are different
Bearings are different
Spindles are different
Some parts houses list different brake hoses.
And, the vacuum booster is different: double diaphragm as opposed to single in JB3.

 

[Rusty Nail]

I was studying about Jb6 to Jb7 differences very recently.
The difference I concluded was in the master cylinder.

The buildup of rear brake line pressure is what triggers the combination valve to begin sending pressure to the front system.

I think the answer was in the question...?

 

[Keith Seymore]

Here's what has me confused, the JB3 and JB5 front brakes are the same other than the thickness of the rotor. In theory, clamping force should be the same, but one will just dissipate heat faster and is less likely to experience brake fade.

What am I missing?

You are correct. No difference in torque output, just heat dissipation.

The calipers are different, of course, to handle the thicker rotors. They must be metered differently internally from the JB3 calipers to reduce the flow of fluid.

They are not. They are the same, by part number. The calipers can handle the difference in thickness, in the same way they handle the difference in thickness as friction material wears.

There is no "metering" in the brake caliper. The only proportioning that takes place is in the diameter of the piston (which is the same in this case).

K

 

[Keith Seymore]

Don't think that I didn't lay awake at night asking myself the very same question.........

Me, too. I probably spent too much time on this last night as well.

Perhaps if we referred to the brake charts themselves it would slake our curiosity, rather than speculating.

The book (all the way back to 1969):

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The chart for 1984:

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[Keith Seymore]

It's a bit hard to condense all that down to something that shows up on a computer screen, but allow me to summarize.

Here are all the "knobs" that you can adjust in the brake system:

Swept diameter
Caliper piston diameter
friction material (front and rear)
drum diameter
shoe width
rear wheel cylinder diameter
master cylinder diameter and stroke
booster size
brake pedal ratio
brake combination valve (meter, split and slope)

When a person says "the difference was the master cylinder", or "the pads are the same" they are neglecting all of the other factors in the equation that may or may not be different.

Also - the apply system (master and booster) do not affect proportioning front to rear; they only affect the force applied to the brake pedal.

So for every combination we produce (1/2 ton, 3/4 ton, one ton, SWB, LWB, 2wd, 4wd) right down to GVW rating, axle ratio and tire size/brand, a vehicle is built and physically tested to develop and verify compliance to federal brake regulations (FMVSS 105 and 208, specifically).

K

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[Ronno6]

You are correct. No difference in torque output, just heat dissipation.



They are not. They are the same, by part number. The calipers can handle the difference in thickness, in the same way they handle the difference in thickness as friction material wears.

I know this to be incorrect. That is how I discovered that I had JB5 front brakes: I purchased the JB3 calipers and they did not fit.I am trying to navigate the GM parts manual to obtain part numbers.

There is no "metering" in the brake caliper. The only proportioning that takes place is in the diameter of the piston (which is the same in this case).

K

I did make an assumption there, as I could not understand calipers with the same piston diameter (I knew that) reacting differently if all else was the same.

 

[Ronno6]

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It's a bit hard to condense all that down to something that shows up on a computer screen, but allow me to summarize.

Here are all the "knobs" that you can adjust in the brake system:

Swept diameter
Caliper piston diameter
friction material (front and rear)
drum diameter
shoe width
rear wheel cylinder diameter
master cylinder diameter and stroke
booster size
brake pedal ratio
brake combination valve (meter, split and slope)

When a person says "the difference was the master cylinder", or "the pads are the same" they are neglecting all of the other factors in the equation that may or may not be different.

Also - the apply system (master and booster) do not affect proportioning front to rear; they only affect the force applied to the brake pedal.

So for every combination we produce (1/2 ton, 3/4 ton, one ton, SWB, LWB, 2wd, 4wd) right down to GVW rating, axle ratio and tire size/brand, a vehicle is built and physically tested to develop and verify compliance to federal brake regulations (FMVSS 105 and 208, specifically).

K

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Yes sir. The master cylinder is common to both systems. The dual diaphragm booster aids in applying the required increase in force to operate the JB5 brakes.
As higher pressure is required to actuate the rear brakes, and the prop valve is common to both systems, how does one account for the "delay" in front brake actuation ??

 

[Keith Seymore]

Dang it. You are right (about the part number. You are not correct about clamp load and the heat dissipation); not sure what I was looking at earlier.

I was working my way up to asking how you knew you had the wrong parts.

Anyway - that's just a confirmation I spent too much time on this last night.

K

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[Keith Seymore]

Yes sir. The master cylinder is common to both systems. The dual diaphragm booster aids in applying the required increase in force to operate the JB5 brakes.
As higher pressure is required to actuate the rear brakes, and the prop valve is common to both systems, how does one account for the "delay" in front brake actuation ??

Prop valve is not common for both

JB3 is 100/300/.27 (meter/split/slope)

JB5 is 100/200/.41

So the JB5 system releases pressure to the rear at a more accelerated rate.

K

 

[Ronno6]

Dang it. You are right; not sure what I was looking at earlier.

I was working my way up to asking how you knew you had the wrong parts.

Anyway - that's just a confirmation I spent too much time on this last night.

K

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Not NEARLY as much time (or $$$$$$$) as I spent on it!

 

[Ronno6]

Prop valve is not common for both.

JB3 is 100/300/.27

JB5 is 100/200/.41

So the JB5 system releases pressure to the rear at a more accelerated rate.

K

Maybe from the factory, but not in aftermarket.
GM 18008084 for the JB3 system does not appear to be available.
GM 18008085 for the JB5 system is.
The prop valve that was in the faulty hybrid setup works perfectly in the homogeneous system.
Or, IS there a prop valve for a mixed system??