Insure that nothing is restricting the pedal from sitting where the MC wants to place it, i.e. not bottoming out on the brake switch or such. If so, that is like the pedal is slightly pressed, which would let cabin air into the booster (how the booster works). If so, insure the output rod to MC adjustment is correct. I can't recall on my M-B, but many boosters have an adjustable tip on the output rod. You are supposed to adjust that until it barely touches the MC piston when bolted up (or even a slight gap). Remove the O-ring and feel for that as you press the MC up to the booster. If you must push in the output rod a bit before it touches, that is likely your problem.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans