I need a brake

[Zed Head]

Second on checking for pressure build-up. That will narrow it down. Take a piece of hose of the size of the bleeder and run it in to a container to avoid spraying.

It seems unlikely that it's mechanical unless your brake pads are so thick that they barely fit between the caliper and rotor and bind when everything expands. Even then, clearance should get looser, not tighter, unless the rotor steel expands more than the caliper steel.

I think that you can have pressure build-up in only the front system since the reservoirs are separate. You might just have a bad MC. Maybe it was assembled incorrectly. Maybe there's debris in the return port acting like a check valve.

[Randalla]

Further update. Limped the car home today and removed the booster to see if I had any available adjustment between the booster and the master cylinder. Based on what I had been hearing, I needed to shorten the actuating rod to allow more free play so the brake fluid would in the master would bleed back, after letting up on the brake. My understanding from what I have read is that if the rod is too long, with too little free play, the brake fluid will expand as it heats up after a number of braking stops, eventually locking the brakes. The pictures show what I found. The reaction disc is not attached to the piston end of the rod but it is in place inside the booster and the piston is flush with it when assembled. As you can see, the rod can not be adjusted shorter, as far as I can tell. The rubber grommet looks is in two pieces (looks to have come apart) and the metal toothed retaining ring has a break in it in one place. When I push in the actuating rod on the pedal side of the booster, it sounds like the rubber seal inside is OK as I can hear vacume but I'm not sure how to test it further.