My '79 300SD had the same problems. I had a vacuum hose broken in half going to the first door lock (drivers) it cracked from opening and closing the door so often right where the hoses go into the door. It was the last thing I checked. The PO had reversed the vacuum check valve that stores up the residual vacuum for the door locks so the system could not leak and affect the other components. the check valve finally failed and I had an open system and all vacuum systems were non-functioning. Have you ruled out the booster itself?

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Originally posted by TXBill:
Tref my 79 240D started out doing the same as yours, brake pedal hard as a rock during short manuevers, slow to turn off etc. Now it won't shut off at all on it's own ever and the brakes are hard most all of the time.
Tomorrow I'm going to get a vaccuum tester because I THINK there is a leak in the passenger side front door lock. I also think there is a leak in the brake vaccuum line from the vaccuum pump because I hear a faint hissing noise from the front of the car while it's running. So, I can't offer any advice at the moment, but rest assured you are not alone with your vaccuum difficulties.

Bill

82 300 SD
79 300 SD
79 240 D

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