Brake question MC / BB

06 BMW 750li

Car had trouble stopping, had to put the brake all the way down to stop at slow speeds. Started to hear a hiss when pressing the pedal. Fluid level was fine, no leaks. Had plenty of vacuum. Signs pointed at the brake booster.

Replaced the brake booster. Had to disconnect the MC, but didnt let fluid drain completly. Put everything back together. Fluid level stayed where it should. With car off pedal was stuck up, hard as a brick wall. Started the car was going to bleed brakes (2 person method). Pedal dropped with a clunk. Sounded like the push rod was clanging against the inside of the MC. Took the MC off to check for a misaligned (bent) rod, every thing looked fine. Put everything back together. Exactly same problem. Clinking sound from the BB when pressing the pedal. Now fluid disappears from the res, no signs of leaks. Pulled vacuum line and there is signs of liquid around the seal. Havent touched it since. I have a new vacuum line with new check valve. Assuming MC must be leaking into the BB.

What should i do at this point? Buy a new BB and MC?

Background, i am an industrial mechanic at a major bottling company. I have built an LS engine, done a internal water pump on a ford V6, and many other things, but never did brake work. Not going to pay bmw 1500 to do this, so if anyone can help i would really appreciate it.


Just to add, that new vacuum line and check valve isnt installled yet. Also from all that i can find this TSW brake booster has a non adjustable rod.

Seems like the master is shot, from your description. I’d replace that first before I’d buy another booster. Just drain out the brake fluid from the booster. Be sure and bench bleed the master before installing. If you lose your booster because of this, then replace it.


Thanks, but looks like i answered my own question.

Anyone have experence with this year 750? There is no space to install the BB and MC attached. But the second firewall is in the way to install the MC to the BB in the car unless you press in it sidways. Anyone know if you can remove that firewall in front of the MC?