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Disc brake lock up

Tiny
MEREDITH BOKENO
  • MEMBER
  • 1967 CHEVROLET CAMARO
  • 15,000 MILES
Installed a front disc brake conversion a year ago. Initially worked fine then hit the brake pulling out of driveway and it stopped so hard that it about through me through the windshield. Re-bled and replaced just about every component never found root cause, worked good again for about six months and just happened again, no collapsed hoses, vacuum at fifteen at idle speed, calipers are retracted, but snug on rotors, fluid, level fine, no leaks front or rear, don't know where to go from here, thoughts?
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Tuesday, May 30th, 2017 AT 5:15 PM

3 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,444 POSTS
Almost sounds like a booster valve issue. Like the valve opens normally at first then the vacuum overcomes the valves return spring and slams the brakes on.
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Wednesday, May 31st, 2017 AT 3:53 AM
Tiny
MEREDITH BOKENO
  • MEMBER
  • 2 POSTS
Thanks, and at this point think you were on the right track. I tested the valve while on the car, seemed to work as it should. Decided to remove it from the booster to check off the car. When I pried it out of rubber grommet, I heard an instantaneous vacuum realease, which I didn't expect. Reassembled started car and has worked fine for last week. Can't rationalize what the root cause is, or how to prevent, but something going on with the valve for sure.
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Sunday, June 4th, 2017 AT 6:38 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,444 POSTS
OK you pulled the check valve (see image). The valve I was referring to is inside the booster itself.
Releasing the vacuum when you remove the check valve is correct it locks vacuum in the booster.

This video explains the internal valving and reaction disc. I believe you have an issue with those valves. The problem is that it's intermittent and not real easy to test because of that.

https://www.youtube.com/watch?v=LThrL8Jjsgc
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Monday, June 5th, 2017 AT 9:46 PM

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