Gradually Locking Front Brakes

[jurven240z]

History: Few years back had to slam on the brake after that lost the booster. Replaced the booster with an aftermarket rebuilt one. The push rod was too long that I had to shorten it to get the master on. Brakes worked great but one thing. They would gradually lock up the front brakes to the point of the engine stalling. The pedal was ferm with no movement. I lost the reaction disk in the booster with all the adjustments of the push rod and never continued the cause of the locking brakes. Now with no firm pedal feeling I went and purchased two master cylinders and still had no firm pedal but the front brakes were locking up. Left it like this till now.

Last weeks project: I went fishing for the reaction disk and fixed that problem. Replaced the master with a 15/16 rebuilt unit. Bench bled and a good bleeding of the system. No problem with the test drive good pedal feel and brakes.

Todays event: Was driving to work this morning and after a mile or two stopping to get gas and red lights the car started slowing down and had to give it gas. Pulled over to a parking lot and the car stalled/stopped on its tracks. Same thing as before brake pedal ferm,hard, no movement. Front brakes locked. I had the spouse bring over some tools released some presure from one of the front calipers (hot) and was able to get brakes back enough to drive it home. Called in to extend my leave by one day to fix this.

So far I figured out the booster is not locking up. How? By removing the master and pressing the brake pedal and I get movement(made sure not to far too lose the reaction disk). Good thing don't need a booster.

Now the master: Could it be bad by not releasing the preasure in the system? Is there a back preasure valve that micht be bad? I just shorten the pushrod and will be releasing the preasure from the front calipers. Will test this before I start removing the master have three spares and two are new rebuilts with less than 20 thousand mile. What do you all think.....5/70 Z

[jmortensen]

The master has to fully relax to release it's pressure. When the piston in the master goes all the way back it opens up a little hole to the reservoir, and the pressure just gets released. If the rod between the back of the master and the reaction disk is too long, the pressure can't release. So as you drive longer the brakes get hotter which expands the fluid which has nowhere to go and basically applies the brakes more and more and more until you stop. You have to have a little bit of play between the rod and the master, 1/8" or so. If not, the problem you're experiencing will result.