My e38 has spongy brakes. when you press down on the brake pedal the brakes engage but when you release the pedal, it does not come all the way up still engaging the brakes. you can release the brakes by lifting the brake pedal with your foot. My brake guy thought it was the master cylinder and replaced same. was better for a time but now back to the same as before the replacement. went back to the brake shop and he disengaged the power booster. hard to brake but the pedal comes right back up after braking. he checked the vacuum pressure which is ok. thinks it is the brake booster. maybe a leak in the diaphram which will not allow the brakes to fully disengage. its an expensive repair, does his diagnosis makes sense?
Thanks