1984 Mercedes 300D

There is a hissing sound coming from the back of the brake booster where the push rod goes in. The sound disappears and vac. actuated stuff starts to work when pressure is applied to the brake pedal. This makes me think that there is a bad seal somewhere in the brake booster. I have tried 3 used boosters and the problem is still here. This is effecting the to the point where the car won't shut off and brake booster isn't working Am I missing something here? Is there an adjustment that can fix this? Am I installing the booster wrong?

On top of that I replaced the master cylinder with a new seal.

Any ideas?

The hissing bothers you?

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84 300SD
85 380SE
83 528e
95 318ic

yes

Make sure there is clearance on brake pedal rod to booster.If it is ok than booster is bad.

Same as answer to your prior question. Insure the brake pedal isn't bottoming out on the brake switch. That would be like someone "riding the brake pedal".

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