Brakes not releasing


Old 02-17-03, 09:56 PM
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Brakes not releasing

I recently had brake work performed..replaced master cyclinder, power boster, front calipers, rear drum cyclinders, etc. The front brakes lock up (won't release) after prolong braking (going downhill) or hard braking. If I let it sit for several hours the brakes loosen up agagain. Vehicle is a 81 chevy 1T dual rear wheel (motorhome) 350 engine.
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Old 02-18-03, 04:33 AM
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There are other possibilities, but often the problem you describe is due to deteriorated front brake hoses. They rot on the inside and the loose pieces act like check valves and keep pressure in the calipers.
Old 02-18-03, 06:30 AM
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Agreed. And whoever performed all that work for you should have known better.

They changed everything else !
Old 02-18-03, 08:11 AM
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I just went through my 77 4x4 brake system. In addition to replacing the caliper hoses I also replaced the rubber hoses on both the fornt and rear axles. I believe you only would have one going down to the rear axle. 4x4's have one on the front axle also. Also have them re-check the free play on the brake pedal, most are adjustable.
Old 02-18-03, 09:05 PM
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If your 'locked" situation occured after replacing the brake booster and master cylinder it sounds to me like you have a problem with insufficient clearance either between the booster (the little rod sticking out of the front of the booster which engages the master cylinder is adjustable inward and outward)and the master or you have no freeplay in the pedal and ther booster. The reason just the front brakes drag is in the the master cyl the primary cup (front brakes) applies first as the piston travels forward thu the cylinder, then the rear is applied. So, if the rod on the front of the booster is protruding to far and is causing the master to apply even the slightest brake pressure to the front it will heat up the fluid in the calipers causing it to expand and thus increasing the fluid press to the front calipers. It cannot release back into the master because in the master the is a valve which when the master is fuly returned to it's non applied position allows the fluid to return to the resevoir. As soon as the piston travels forward this fluid return passage is shut off.

First make sure ther is a little freeplay in the brake pedal... then, next time this problem occurs jack up and raise a front wheel off the ground. Verify it is locked. Then loosen the master cyl where it mounts to the brake booster and pull it away from the booster an 1/8 of an inch or so wedging it out from the booster with a flatblade screwdriver if needed. If the wheel nows spins freely you have just confirmed it is a master cyl/ booster clearance problem. If not, then loosen the brake lines on the master, fluid should drip out, not squirt out, occasionally the valve in the master is bad (and it doesn't matter that it is new!!) and never allows the brake pressure to bleed off when released. If it rolls free now the master is bad. If it is still locked, now loosen the bleeder valve on the caliper. If fluid squirts out and the calipers release thier hold on the rotors the flex hoses are bad. Hope this helps>
Old 02-20-03, 09:39 PM
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Thanks for the suggestions. The mechanic is now claiming the proportioning valve is at fault. Its the only left to replace. Naturally he wants more money.
Old 02-21-03, 03:27 AM
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Fine. Tell him if the brakes do the same thing after his repairs, that he's going to give it to you for free .
Old 02-25-03, 09:46 PM
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Replacing the proportional valve did not solve the problem (I did not pay for the valve) although its seems to have slightly improved..I don't have to wait as long only about 5 minutes for the brakes to release after shutting off the engine. Mechanic claims there is no adjustment on the hydro boost to master cyclinder. Comes fixed from manufacturer. There are 3 flex hoses in system: LF, RF and near rear wheels. LF was replaced when calipers were done. RF wasn't. I don't observe any pulling or indication of just the RF locking up; both front wheels appear to lock up the same amount. I'm about ready to give up and start over with someone else perhaps a truck brake specialist.
Old 02-26-03, 02:47 AM
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I agree with RKINGDAVE his diag is on the $. It make the most sense because problem started after repair


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