I hope this helps you some. I had a very similar problem recently with a "76 model Corvette, mine turned out to be a faulty brake master cylinder. The vaccum brake booster may also be suspect on your Mercedes but I'd start with the master cylinder. I wonder if someone here has a test method for the msater cylinder?
Al

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2007 Mini Cooper 4K miles
1993 300E 2.8 120K miles
1961 Morris Minor 42K miles
1965 Morris Minor 23K miles
1997 Ford F150 235K miles
1987 Ramcharger 202K miles
1984 Chevy C-30 farm truck 88Kmiles
1968 XKE 59K miles
2001 Harley Softtail
2003 Harley Roadking