My 84 733i did the same thing about a year ago, it wouldn't pass the German inspection and I had to take it to the BMW dealer here to gert it fixed because I was stummped. It wasn't the master Cyl. Depending on the breaking system, if it is part of the hydrolic system it may be the regulator valve on the accumilator--or the acumilator itself if that has not been replced. There is also a sneaky filter in the system that needs to be replaced--I didn't know about that oen until the dealer told me about it and showed me the clogged one.