Hello Brickboard group. My problem / question is about the symptoms of a bad brake booster on a '91 745 turbo with ABS. I have put a new (rebuilt) and a new (aftermarket) master cylinder on the car. I have isolated the MC both times by pluging the line outlet holes. ......... Still the brake pedal will sink all the way to the floor under medium or heavy pressure. The original symptoms that lead me to replace the MC was a very soft brake pedal (even after much bleeding of the lines) . Everywhere i look online says a bad brake booster makes a hissing noise when you press the pedal, and the brakes feel extra firm, nothing about the pedal going all the way to the floor under steady pressure. Has anyone had the symptom of brake pedal going all the way down fixed by putting on a new brake booster ?

Thanks for any input, ...... this is really starting to get me down.