I had an interesting problem today. As I drove away from the house, the brakes were very hard to push - lit felt like no vacuum assist. Didn't think that much about it, thought I'd check it out when I got back home. When I got to the store, the engine wouldn't shut off with the key - shut it off with the STOP lever. When I drove from the store to my next stop the brakes started working and when I stopped, the engine shut off by the key. Left there and it went back to the original problem - hard brakes and didn't shut off when I got home.
The problem - the valve on the vacuum pump. I disconnected the metal line from the vacuum pump (19mm wrench) and removed the valve (22mm). Shined a light down into the top of the vacuum pump and yep, the valve was self destructing.
Called the MB dealer - the valve was $83, but with the MBCA discount, $41.

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Houston, Texas
1984 300CD 235K miles, Elvira, one owner
1987 300TD 180K miles
1974 TR6 78K miles Sarah Jane, one owner
OBK #27