Brakes slowly locking up while driving

[Minusartifact]

82 e21 with 95 e36 m3 motor and transmission. Wilwood 4 pot calipers in front, rear drum to euro disc conversion in the back (from what ive been told), bc coilovers, etc. Everything on the vehicle is either brand new, reman, or rebuilt as part of a full chassis restomod rebuild

We've been trying to chase a problem with the braking system. While driving, all 4 of the calipers are slowly locking up. When the brakes are cold, there is little to no drag on the brakes. But as you drive and warm them up, they slowly lock up until the vehicle can't move anymore. We've done the equivalent of about 5 brake flushes on the whole system, and it helped, but didn't fix. When the brakes are locked up, there is residual pressure in the lines from what we can tell. if we crack open one bleeder valve, all 4 of the calipers become free. Everything from what we can see is operating correctly. Vehicle has reman master cylinder on rebuilt brake booster, new braided lines on calipers, reman residual pressure valve. We're completely stumped as to what is going on.

[Wilmo]

That’s a cool setup! Would be nice to see some pictures and introduction of the car!

But first the problem. I was thinking towards the brake booster and would suggest to remove the vacuum temporarily. But the same information would be to measure the shaft going in the booster sticking out from the firewall to the inside. When that moves, the problem is between main cylinder and brake pedal. When it stays it is in the rest of the system.

The only other way to build pressure is from one of the other four callipers. Trapped air to expand by temperature… my lazy chair engineering says that that wouldn’t stop the car.

[nic65]

it happened to me too, it was a problem with the brake booster

[Jeroen]

Heard that before, master-booster combo mismatch. Would defo start there.

[nic65]

exactly, when the brake booster does not guarantee sealing, every time it feels depression it loads the brake master cylinder again

[Minusartifact]

I'm not doubting anyone here, but before I go spend $200 on a new one, I want to make sureits the booster. I don't want to throw parts at this vehicle as it's not my car. It's a friend's car that he left for me to fix when he moved cross country as he couldn't take it with him. He would be paying for everything, and I don't want to waste his money

1: Is there a way to test to make sure it's the booster and not something else

2: with everyone saying it's the booster, I feel like it would lock up if I continuously pressed the brakes at a stop with the engine running, but it doesn't do that.

3: if I do replace it and the problem persists, what could it be then?

[Wilmo]

Take the vacuum off (and plug the hole) and take a drive. Be prepared for really heavy brakes, but without vacuum, the booster will not do anything.

[kam]

I had something similar but not quite the same year back on an M10 1800 which had been turbo charged. If I was crawling in slow traffic or at a standstill with the engine running for too long, the brakes would lock up until I turned the engine off, let it cool and all was good again, If I was in moving traffic and airflow then it wouldn't lock until I was in stand still traffic and heat built up under the bonnet. I was told by a mechanic at the time the brake fluid may have been boiling due to the proximity of the manifold in that car, but not sure how true that was. I never got to the bottom of it as someone bought the car from me.

[nic65]

if the liquid boils it produces air, the air is compressible, it is difficult for the brakes to remain blocked

[Minusartifact]

Update:
We replaced the master cylinder and brake booster, and that hasn't fixed the issue. Still locks up after about a mile of driving, and even if it sits in the heat, brakes are locked up. What else could be causing the issue

[Minusartifact]

Update:
We replaced the master cylinder and brake booster, and that hasn't fixed the issue. Still locks up after about a mile of driving, and even if it sits in the heat, brakes are locked up. What else could be causing the issue