Loss of all brake pressure, losing no fluid, bleeding only temp. fix

[euroshark]

Do all E30s have a vacuum booster? I'm definitely more of an E24 and E28 guru... Check that the hose from the manifold to the booster is not collapsing under the higher vacuum pressure that you are probably experiencing at highway speeds due to higher RPM.

[hamann318is]

Hey y'all, I'm still having the same sort of brake problem that I posted about earlier.

The story is that after taking the car (91 318is) on the track, I had a very mushy pedal. I took it home, bled it, and all was well... until I took it on to the highway. I lost pressure and had a mushy pedal again. I figured it was the master and purchased a new master cylinder. Bled the brakes, and all was well until I had it on the highway. I lost pressure again.

After consulting the forums, I had a shop bleed the brakes. The same situation resulted. Great pressure... until taking it on the highway where I loose all pressure. I've done this two more times, and nothing seems to resolve the problem.

Simply, the car has a new master cylinder and looses pressure when I have it on the highway. The pressure does not come back; I have a mushy pedal until I bleed the brakes again. More importantly: the car does NOT lose any fluid. There are no leaks; the tank on the MC is still at MAX.

I don't know what is wrong. Could I have a rubbish new master? I don't think it is the brake booster, the pedal is mushy - never rock hard. The pads have decent life left. Any help would be appreciated in solving this problem! Thank you in advance!