Tricky Brake issue, help needed brake gurus

If it were me, I would bypass the front valve with a temp line to eliminate one possible cause.

 

If you let it set for awhile and wheels are free.
Now if you can drive it with out using the brakes maybe hard to do were you live,and see if the brakes do it without using them.
Those rubber brakes lines maybe even s steel if they get damaged they will do that.
Never pinch them.

 

We just shimmed the master away from the booster about 1/4" (overkill) and took it for a nice long test drive. It seems like it's working great for now but I've had it work well before. Where going to lessen the shimmed amount and drive it again after letting it set and see if anything happens from heat sink. Wish me luck!

 

You've got it!

 

So far so good. The master seems to be leaking now out of the front seal. We'll get a new one on there and see what happens.

It looks like the HAMB has come through again. Thanks everyone.

 

It totally sounded like it was bound up, similar to mis-adjusting the push rod on any master cylinder. Keep an eye on it, maybe drop the shim to an 1/8" eventually.

Bob

 

Isen't there a tool to set that spacing". I just looked in Speedway Motors and did not find it.

 

If this were true, every time the brakes were applied, and heat was induced from the friction, the brakes would only work once before locking up. Brake fluid is fairly stable. Only when it is overheated(boiled) does it change from liquid to gas, which is compressible. Any change in the brakes systems volume (brake wear, pad kickback, etc) is easily compensated for. And the brakes are only under pressure normally when the pedal is applied. With the exception of the delay return valves installed in drum systems to prevent air from being sucked in past the seals due to the springs retracting the shoes quickly.

More than likely;
1. the pushrod length is too long
2. the pedal assembly is binding.
3. an issue with the MCs compensation port

There is a tool, OTC and Lisle sell an adjustable unit for multiple applications, but often you will see a pre-zized tool for only one application.

For better visuals on measuring/adjusting the pushrod length...
http://www.maximummotorsports.com/content/install/pdf/brakes/MMBAK-1r1.pdf

 

Mike great post. I'll save this for future reference.

 

Sounds like you found the problem. I wouldn't reduce the shims unless the pedal play is excessive - don't mess with success... If it happens again with the current shims, I'd replace the master cylinder.

 

I've got a very similar issue with my 55 Chevy Gasser. It's got Vette 4 wheel disks (75) and power booster/master. I've been messing with rod length and spacers to get correct pedal travel. New master and calipers. When I first start out, pedal feels great, stops on a dime. If I do a lot of stop and go, the pedal travel tightens right up and drags the brakes hard. Not much pedal travel at all, pedal is hard. If I shut the car off and restart (less than 20 seconds) pedal feel is back to normal with no brakes dragging.

Could an odd high vacuum condition or possible leak cause the booster to apply the brakes?

Thanks!