'77 3/4 ton rear brakes hanging up

[corn]

A friend stopped by at work today saying he was out looking for a new proportioning valve because his rear brakes keep hanging up. RR first then the left side. I told him it was probably a wheel cylinder problem but he said they are good. Any ideas here because Im told they dont make proportioning valves for these trucks anymore & I told him Id ask the pros here for help.



I tried the search but couldnt come up with anything...Thanks in advance...

[cochino12]

My 86 does the same thing and I was told it was a wheel cylinder, maybe check them again?

[79blazer]

I have had this problem too! One rear wheel will lock up for a day or two, then it will stop for a couple days, then the Other rear wheel locks up. This has happened on my 85 caprice, 88 c2500, and 79 blazer. The lock up situation happens just by barely touching the petal, and only at slow speeds. Under 10 miles/hr. It does it mostly on gravel, but can on pavement with a quick jab of the brake petal. Also there is feedback through the foot, kind of a sticking feeling or something. I don't know for sure what it was, (never fixed it) but I figured it was the prop.

[spudz]

What USUALLY causes this is a bad wheel cyl or leaky axle seal.. When the wheel cyl or axle seal goes bad, the brake fluid or axle fluid contaminates the shoes and causes them to grab prematurely. Kinda like this.... put on a rubber glove, put some motor oil on your glove, go to your wifes freshly cleaned picture window, firmly push on your hand and scoot it along, yes it will slide but it will catch and release kinda like a windshield wiper does on a dry windshield. NOW if you find you in fact do have a leaky wheel cyl or axle seal.....dont just think you can replace them....once your shoes are contaminated, thats it.....you have to replace them, ..... But since both sides are doing it, it probably is the prob valve and he just doesn't have his rear brakes adjusted evenly since one side is grabbing first.

[James McClure]

Spudz got it right with this added in for good measure. Chevy trucks have always had this problem, especially 3/4 ton and up. It gets real bad if truck sits for a few days. Mine has always done this. After the first stop it's fine. The weight limiting valve doesen't seem to help at all. It's not the prop valve, that only controls front\rear, not L/R and if thyere were a problem with the front or rear brake the light would be on. jim

[powerdriver 1958]

If he has a full floater rear did he pull the axles and look?In this weather all it takes is a couple of drips,Might not show just looking at the backing plate.Since both are leaking he needs to check to axle vent, If its plugged back pressure can build and make a seal leak..........

[James McClure]

If there is a question as to which type of fluid it is if there is a real mess back there, smell might be the first clue, then try rinsing the mess with water. If it disolves in water it's brake fluid, if not it's gear oil or both fluids. jim