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Old 09-23-2012, 07:33 AM
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Brakes appling themselves Help

I have a 57 Studebaker pickup with 4 wheel disc brakes I recently purchased. It had a leaking brake booster that I replaced. Then the problems started. Ocassionally the brakes don't release when applied and the pedal doesn't reurn all the way back. If I pull the pedal back with my hand then the brakes release. Also the brakes will apply themselves and again if I pull the pedal back they release. I have added a strong return spring on the pedal with no results. Clearance between booster and master cylinder has been checked and gives about 1/4 inch freeplay. I checked the m/c and fluid was overflowing and I reduced it to about 1 inch below the top with no resulting change. Any ideas

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Old 09-23-2012, 08:52 AM
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Maybe something here.
Classic Chevy, Chevrolet, GMC, Ford technical articles
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Old 09-23-2012, 09:02 AM
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Sounds like a defective booster since the problem started after replacing it. Was the booster new or used?
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Old 09-23-2012, 09:06 AM
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heat build up ?

My son had a nash Metro that we swapped in a 1600 pinto and 4 spd, the brake lines were too close to the exhaust, boil the fluid ,
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Old 09-23-2012, 09:40 AM
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New booster. Why do you think it's the booster other than it being replaced? I did the booster tests shown on a link in this post and it checked out
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Old 09-23-2012, 10:31 AM
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Are you running residual valves ? If the MC resevoir is higher than the calipers by quite a bit it can prevent them from releasing correctly. Combine this with a small amount of heat build up it can cause intermittent braking pressure. I cannot remember the particulars , but I know of at least one case where a NEW booster was at fault. Is the pedal to booster connection allowing a complete return? Just ideas.
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Old 09-23-2012, 10:31 AM
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Quote:
Originally Posted by Stude57 View Post
New booster. Why do you think it's the booster other than it being replaced? I did the booster tests shown on a link in this post and it checked out
I'm just using logic in that the issue did not start until you replaced the booster. Just because it's new, don't mean it's good. You never know how long that new booster set on a shelf somewhere. If it checked out and you're satisfied it's not the booster, then there is a problem elsewhere of course.
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Old 09-23-2012, 12:24 PM
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Quote:
Originally Posted by S10 Racer View Post
I'm just using logic in that the issue did not start until you replaced the booster. Just because it's new, don't mean it's good. You never know how long that new booster set on a shelf somewhere. If it checked out and you're satisfied it's not the booster, then there is a problem elsewhere of course.
This is also my vote.
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Old 09-23-2012, 04:04 PM
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I believe the booster pushrod to the master cylinder is out of adjustment, or the pushrod from the pedal to the booster is out of adjustment, IE. too long in both cases.
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Old 09-23-2012, 05:21 PM
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Quote:
Originally Posted by 65G10 View Post
Are you running residual valves ? If the MC resevoir is higher than the calipers by quite a bit it can prevent them from releasing correctly. Combine this with a small amount of heat build up it can cause intermittent braking pressure. I cannot remember the particulars , but I know of at least one case where a NEW booster was at fault. Is the pedal to booster connection allowing a complete return? Just ideas.
No residual valves. I followed the booster check in a link provided by RWENUTS and all test show the booster to be good. I can get the system to fail to release the brakes in the garage so I don't think heat is the problem. I start with 1/4 to 1/2 inch freeplay. Start the truck and put it in gear and start applying the brakes to hold the truck. After about the 5th brake application the brakes fail to release. I just pull up on the pedal and they release.
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Old 09-23-2012, 05:24 PM
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Quote:
Originally Posted by enjenjo View Post
I believe the booster pushrod to the master cylinder is out of adjustment, or the pushrod from the pedal to the booster is out of adjustment, IE. too long in both cases.
I put a dab of grease on the end of the booster pushrod and bolted the m/c tight and kept doing this until I didn't transfer any grease and then turned the pushrod in one turn. I'm not sure how to adjust the pedal to booster pushrod.
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Old 09-23-2012, 05:31 PM
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Quote:
Originally Posted by S10 Racer View Post
I'm just using logic in that the issue did not start until you replaced the booster. Just because it's new, don't mean it's good. You never know how long that new booster set on a shelf somewhere. If it checked out and you're satisfied it's not the booster, then there is a problem elsewhere of course.
I agree that just because a part is new doesn't mean it can't be defective. I did the 3 tests in the link posted by RWENUT in this thread and the booster passed. I also verfied the check valve works and I have over 20in of vacuum. I'm just trying to avoid throwing parts at the problem.
I have now learned I can duplicate the problem at will. I start the truck and put in in gear. I keep tapping the brakes to keep the truck from moving and on about the 5th brake application they fail to release and I can tell that with each brake application from the beginning the brakes release less and less. After they fail to release I just pull up on the pedal and the truck starts to move.
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Old 09-23-2012, 06:39 PM
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Doesn't make sense it will release when you pull the pedal up by hand but not when it's pulled up with a spring.
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Old 09-23-2012, 07:48 PM
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Quote:
Originally Posted by 68NovaSS View Post
Doesn't make sense it will release when you pull the pedal up by hand but not when it's pulled up with a spring.
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Old 09-23-2012, 08:09 PM
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Quote:
Originally Posted by 68NovaSS View Post
Doesn't make sense it will release when you pull the pedal up by hand but not when it's pulled up with a spring.
Apparently the spring is not strong enough to pull the pedal back. The spring is strong enough that when I put it on I needed vise grips to pull it back to its bracket. I read in another thread about a simular problem and a poster suggested that brake fluid may be leaking past a seal in the m/c, so I may change the m/c in the next few days. I was trying to determine the reason before just replacing parts.
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