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Old 09-08-2011, 10:30 AM
10,869 posts, read 41,139,178 times
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"something fishy here" ....

I've never seen a brake booster fail in such a way as to apply pressure to the master cylinder. The usual failure mode is that it doesn't "boost", or gets mushy in assisting the brake pedal application.

But ... I have seen a brake hydraulic system that really needs to be flushed, and when pistons in a caliper were retracted to accept new brake pads, they pushed a bunch of junk back to the brake master cylinder, causing it to stick. So, you could apply the brakes and the master cylinder piston would not completely retract to uncover the return port after you released the pedal, allowing the hydraulic pressure to remain on the pistons.

I have also seen internal failures in the flex hoses to the calipers, where twisting the hose had caused a wall separation inside. A flap of hose then acted as a one-way check valve on a caliper, similar to the "twisted hose" failure mentioned by others.

The OP doesn't mention a pull to one side in the steering, which would be the result of only one front caliper locking up. This leads me to favor the sticking master cylinder as the failure area, because it sounds like both front brake calipers are dragging.
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Old 09-08-2011, 10:31 AM
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Originally Posted by Gimme3steps View Post
Quickest way to confirm what they're telling you, assuming the front wheels are still locked up, is to have both front wheels off the ground and remove the nuts holding the master cylinder to the booster and separate the two. Do not disconnect or loosen any brake lines. If there's a problem with the booster this will isolate it from the brake system and the front wheels should free up. If they don't, then you have other problems.

One thing I've seen over the years is the caliper gets rotated while it's off and then installed with the hose twisted. You can apply the brakes but they won't release.

Whatever the problem is, make sure they replace the pads again after the fix. If they've been that hot then they shouldn't be re-used. If they were a brand name pad they should be able to send them back as defects at no charge to you.

Good luck. (what's the year, make and model?)
Thanks for this info. I am learning something new everyday.
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Old 09-08-2011, 11:18 AM
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Originally Posted by brandonc27 View Post
I'm having the exact problem with my 92 prelude my brakes lock after driving for about a mile and hot weather . The pedal gets really hard and the car slows down and when I step on the gas I have to go over 5000 rpm and won't go over 30 , I replaced pads, rotors, calipers , and new brake fluid and still lock up. Can anyone help?? What was your solution
Did you do the work yourself? If you paid someone to do it, was it some white van mechanic, or a shop? Wrong diagnostic is fairly common with DIY mechanics. But a shop should have done a better job at identifying the problem the first time.

Sorry I have no solution for you cause I am not even close to being a mechanic.
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Old 11-12-2011, 07:45 AM
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I have a honda accord 90 and I cant remove the brake booster.. Everthing is loose and removed but its no room to maneuver it off.. Theres like a harness in my way, am I supposed to take that off too?
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Old 11-12-2011, 08:35 AM
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http://www.google.com/url?sa=t&rct=j...vUshG3dqbwTher you go, this should help.
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Old 11-12-2011, 08:54 AM
Location: San Diego, CA
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Does the car have a proportioning valve? This sets the amount of braking effort front to rear.
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Old 09-27-2012, 01:21 AM
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I'm having the exact problem with my lexus es 300 the brakes lock after driving for about a mile or so and also in hot weather. While i am driving suddnly the car slows down and when I press the gas very hard I am not peeking any speed and then the car suddnly won't move and the brake padal feels realy hard, but the engen is on and no light shows on the dashebord. The engen is running but when I put it on drive it won't move and also it won't move when i put it on reverse. I also replaced pads, rotors, calipers , and new brake fluid but still i have the this problem. This actualy happened when I tooke my car to a machanic to change the brakes beacuse they were woren out then afther that this problem accured. I tooke my car to different auto reaper shops but seems like they do not know what is wrong with it beacuose no light comes on, on the dashebord. the machanic told me that three weels lock when thes happens. I have spend so much money because of this and i am not able to drive my car like this becuose it is very dangeres to stop suddnly on the rode. Can any one please help me. Please let me know what you did when you had this problem.
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Old 09-27-2012, 09:55 AM
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Do the wheels actually lock up or does the car just not have enough power to pull itself? Stop on a hillside the next time it acts up and put it in neutral and see if it rolls. That'll tell you if the brakes are locking up.

If it's the latter, it sounds like your catalytic convertor is clogged. No power and a hard brake pedal are the 2 main symptoms. Easy to check with a vacuum gauge or a back pressure tester.
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Old 09-27-2012, 01:56 PM
Location: Dickinson, North Dakota
105 posts, read 391,781 times
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Hearing the posters comment about only being able to drive a mile or so down the road and than brakes lock up has me wondering. I do custom builds on the side for pretty much anything that goes fast.

Have you currently done any other modifications to the vehicle, specifcally brake line routing or exhaust.

I had a friend who had his custom motorcycle worked on at a local shop. Had new braided lines installed, calipers, the whole 9. He calls me up with the same issue he would run the bike a ways down the road his rear brake would lock up.

I dug into it and found that when they put the new lines on they routed them in a way to make them less visible. However, by doing so it was closer to his pipes. Still plenty of room between the line and pipes. I rerouted the cable away from the pipe and the issue was solved. His exhaust was heating up and boiling his brake fluid in his line causing the brake to lock up.

Just a little first hand knowledge and story to throw your way.
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Old 09-27-2012, 03:31 PM
Location: What use to be the South
441 posts, read 1,238,646 times
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From the info provided, I would look hard at the front hard line. Make sure it was not crushed by a lift pad or pry bar. Follow from the master to the split off. A single blocked brake hose will cause a pull to the opposite side of the blockage. Hit the brakes hard enough and it will pull one direction and then back to the other. The second thing and has been mentioned is the proportioning valve. The way it is designed, if you have a failure, it will essentially shut off the effected portion and apply everything to the other end. Now, with the pistons pushed back, the first couple of strokes of the pedal are going to activate the proportioning valve to the full rear position. It could be a case where the valve is stuck with the fronts all but shut off. Enough pressure could be applying the front disc but not be able to return, keeping pressure on the fronts.
It you can, I would jack the car up after sitting and try to rotate the tires. Now, apply the brakes real hard and return to the tires and try to rotate them. If one or more will not turn or drag, break the bleeder loose and see if this frees it up. If so, you have a blockage. You can break the line loose in sections (applying pressure each time) to locate the section or component with the problem.
Last but not least, and this would not cause the brakes to hang, but just make sure the vac hose to the booster is in tact and connected.
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