Rear Brakes Locking Up

[Andronikos]

I have a '86 half ton 2wd. It has the small brakes, 2" shoes in the back and 1" thick rotors in front, power assist. The rear brakes lock up with just moderate pedal pressure and the fronts don't lock up no matter how much pedal I give it. Also the rear drums make a hell of a noise as I come to a stop. Literally everything is new. MC, steel lines, rubber lines, calipers. rotors, pads, shoes, and wheel cylinders. I have bled all 4 corners and get fluid at all, so I don't think the "proportioning valve" is an issue, but that's the only thing that isn't new besides the booster. I have about 600 miles on all this stuff and the front rotors still look almost new. What the hell am I missing? This can't be normal.

Thanks in advance

 

[Edelbrock]

If the only thing that is old is the proportioning valve, why not replace it?

 

[Andronikos]

If the only thing that is old is the proportioning valve, why not replace it?

I get fluid at all four corners when bleeding. I don't see how it could be the issue, but if it could be the problem can you please explain how?

 

[83Stepper]

Sounds like either the proportioning valve, or you got your lines crossed somewhere between the master cylinder and proportioning valve. Most of your stopping power comes from the front brakes. With the rears locking up and the fronts not engaging makes me think that's your problem.

 

[Goldie Driver]

Just my .02 from personal experience, but I would not change the valve as your replacement is going to be Chinesium. If you do change it , hang on to the old one. One thought might be that the valve is not centered. With a helper, try cracking the rear bleeder screws and push the brake pedal down and hold it until the screws are tightened. The idea is to get the valve to move the other way - if it is indeed off center.

 

[Andronikos]

Sounds like either the proportioning valve, or you got your lines crossed somewhere between the master cylinder and proportioning valve. Most of your stopping power comes from the front brakes. With the rears locking up and the fronts not engaging makes me think that's your problem.

I have the rear port of the master, the small line, feeding the front brakes, and the front port of the master feeding the rear brakes. Is that right or do I have it backwards?

 

[Andronikos]

Yes, I have my doubts about new valves too. The way you mention about re-centering the valve is how I bled the rear brakes. Wouldn't that essentially do the same thing?

 

[83Stepper]

I have the rear port of the master, the small line, feeding the front brakes, and the front port of the master feeding the rear brakes. Is that right or do I have it backwards?

That's right, rear of the master feeds the front brakes and front of the master feeds the rears. Check to make sure your proportioning valve is centered.

 

[Goldie Driver]

Yes, I have my doubts about new valves too. The way you mention about re-centering the valve is how I bled the rear brakes. Wouldn't that essentially do the same thing?

Yes and no as I assume you and your helper did one wheel cylinder at a time. I am saying to open BOTH rear bleeders in an effort to emulate a rear failure and make the proportional valve spool shift over. They normally will recenter once the fault is corrected and a hard press of the brake pedal is applied but it may be sticky.

 

[Blue Ox]

There's always confusion about what these valves are and what they do.

It's called a combination valve because there's three different valves in there.

The front brake portion is a residual pressure valve. It allows the drums to apply first, then once a couple hundred psi have built up it opens and allows fluid flow to the fronts. The back end of it is a proportioning valve. This limits pressure to the rears once a certain level has been reached to prevent the rears from locking up prematurely. The center piece is the switch for the warning light. If the pressure is unequal between the front and the back due to a leak it will shuttle over to the low pressure side and illuminate the warning light.

Many people say that it closes the fluid flow to the failed side. There's nothing in the diagrams or my experience that confirms this. Every brake failure I've experienced has lead to the dumping of fluid from the failed side and a low pedal. YMMV.

 

[Goldie Driver]

There's always confusion about what these valves are and what they do.

It's called a combination valve because there's three different valves in there.

The front brake portion is a residual pressure valve. It allows the drums to apply first, then once a couple hundred psi have built up it opens and allows fluid flow to the fronts. The back end of it is a proportioning valve. This limits pressure to the rears once a certain level has been reached to prevent the rears from locking up prematurely. The center piece is the switch for the warning light. If the pressure is unequal between the front and the back due to a leak it will shuttle over to the low pressure side and illuminate the warning light.

Many people say that it closes the fluid flow to the failed side. There's nothing in the diagrams or my experience that confirms this. Every brake failure I've experienced has lead to the dumping of fluid from the failed side and a low pedal. YMMV.

I think the way to look at it is that it isolates the good side and you won't lose the fluid from that side of the system. When mine failed not too long ago ( aftermarket replacement) that is what happened. I did have a low pedal as I was only stopping on the rears at the end which I think is due to the empty chamber in the master cylinder and the lack of resistance caused by that lack of fluid.

In my case the seal on the drivers side of the valve - where the rubber plug is - failed and was pumping fluid every time I hit the brakes. I thought the pedal felt funny on the way in to work and the pool of fluid underneath the truck once it was parked was the proof. Glad I had saved my OE one.

 

[Andronikos]

Yes and no as I assume you and your helper did one wheel cylinder at a time. I am saying to open BOTH rear bleeders in an effort to emulate a rear failure and make the proportional valve spool shift over. They normally will recenter once the fault is corrected and a hard press of the brake pedal is applied but it may be sticky.

OK. I'll try that. Thanks

 

[Andronikos]

There's always confusion about what these valves are and what they do.

It's called a combination valve because there's three different valves in there.

The front brake portion is a residual pressure valve. It allows the drums to apply first, then once a couple hundred psi have built up it opens and allows fluid flow to the fronts. The back end of it is a proportioning valve. This limits pressure to the rears once a certain level has been reached to prevent the rears from locking up prematurely. The center piece is the switch for the warning light. If the pressure is unequal between the front and the back due to a leak it will shuttle over to the low pressure side and illuminate the warning light.

Many people say that it closes the fluid flow to the failed side. There's nothing in the diagrams or my experience that confirms this. Every brake failure I've experienced has lead to the dumping of fluid from the failed side and a low pedal. YMMV.

OK. So you're saying concentrate on the proportioning valve. I'll try that.

 

[Blue Ox]

From the manual:

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Doesn't provide a lot of answers, but at least you can see what all is in there.

 

[DoubleDingo]

I have the rear port of the master, the small line, feeding the front brakes, and the front port of the master feeding the rear brakes. Is that right or do I have it backwards?

Maybe mine is wrong, but it is as it came from the factory as far as I know, and the small line is hooked up to the front port of the master cylinder. I was always under the impression that the rear port is rear brakes, front port, front brakes. When I swapped in front disc on my 65 C20, I did it just like this and never had any braking problems.

Granted my 81 is a C20, I think the small line is supposed to go on the front port. Here is a pic of my C20 master cylinder...

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