Strange brake problem, It's back!

[airwolf]

This brake problem is going to make my head explode ! I posted about my locking rear brakes a couple of days ago and got responses pointing to a bad master cylinder. I bought a new one and the problem seemed to go away. No problems for a couple of days and today, it's back. It was fine until I started pushing the car a little bit. I punched it a few times and sure enough it started to slow down again. I pulled over and the rear brakes were smoking again! Could smell the hot brakes. When I put it in gear it won't even move without stepping on the gas! I shut it off and didn't do anything else for about 20 minutes and restarted, then everything was fine again. What the $#%^ is going on with this thing??

 

[WSLN 6]

Just a shot in the dark, but is it possible that the proportioning valve is holding the pressure to the back brakes when it gets hot?
Not sure if that's possible, maybe someone could step in.......

 

[John Larkin]

Is it possible that you have the star adjusters in backwards and they are tightening the shoes as you drive? Just a thought? They should be toward the rear of the car on both sides I'm 99% sure.

 

[airwolf]

brake problem

Checked the star adjusters, replaced proportioning valve and went for a drive. Everything was good for about an hour. Then it did it again! I did notice that it doesn't happen if I stay out of boost! As soon as I got on it enough to build about 10 lbs of boost the brakes locked up within about a half mile. I released the pressure at the m/c rear brake line and tried again. Drove around town for another half hour with no boost, brakes were fine. Got on it again up to 10lbs and guess what!!?? Within less than a mile they locked up again! I've replaced the check valve at the booster, m/c and porportioning valve. I have a billet vacuum block with the fitting for vacuum brakes. The brakes work very well when they don't lock! What is the relationship between boost and the brakes?? What else can I check???? I'm totally lost now! Thanks for any help or suggestions.

 

[Chuck Leeper]

BOOOOOOSTTT!

Sounds to me as tho the check valve is not working, and you are pushing boost into the booster cannister, and applying the brakes.
I'd test the check valve w/ some serious "boost", aka, the air compressor in your shop....

 

[SinistrV6]

I realize this post is a little older, but I just came across it in a search on vacuum conversions. If you haven't solved this problem yet, may I suggest that you have a brake line that is too close to the exhaust at some point on the car. Not getting hot enough to vaporize the brake fluid but enough to make it expand and apply the brakes. Just my .02. It seems to come and go (by your description in your posts) with heat build up in the car. Boost obviously aggravates the situation (more heat).

Just a thought,