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brakes not disengaging on a 1985 GMC Sierra



I have a 1985 GMC Sierra 1500 with power brakes and front disc and rear drum. It has the regular power booster and not the hydro-boost system that some of them had. One night when I was driving along the truck suddenly started jerking and not wanting to go and I realized that the brakes weren't disengaging. When I stopped the front brakes were overly hot, but I had managed to stop before they started smoking. The rear brakes seemed normal, but the truck hadn't really gone with the brakes engaged for me to be 100% sure. My first thought was that the master cylinder was sticking. After beating on it and finally unscrewing one of the brake lines at the master cylinder just enough to release pressure the brakes started working normally again.


Not too long after that it started doing it again. I figured out that releasing the pressure would get the brakes to release, but as soon as I pressed the brakes again they would start sticking. They would keep engaging a little more everytime I pressed the brake pedal until the brakes were almost fully engaged.


I finally replaced the master cylinder thinking this was the problem. However, it was still doing the exact same thing. I've tried to think of other things it could be without much luck. It's not the parking brake since it's the front brakes that are locking up. I thought about the calipers, but I can't imagine both of them having the exact same problem at the exact same moment. I wondered about the combination valve, but I don't know much about it and from what I've read on it I can't figure out how it could be the problem.


Hopefully that was detailed enough for someone to figure out what the problem is. I appreciate any information anyone can give me.

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Did you check the shaft from the vacuum booster yet? It's possible that the booster is sticking, so the booster is still applying pressure to the master cylinder even though you let off the pedal. That's all I can think of, hope it helps.


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I would check out the front calipers. I'd make sure the caliper slides/bolts, mounting surface and bushings were clean and lubed and I'd also check that the caliper piston could be pushed back into its bore smoothly and without great effort. Also check front flex hoses for deterioration....just my 2 cents on this strange problem.


I've had sticking caliper problems with both my 77 Seirra and my 82 Scottsdale.

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