You are going to need to familiarize yourself w/ the MB diesel tranny vacuum system. I am sure that your vacuum set up will not be too different than that of w123 diesel of the same age. Search the forum here. There is a ton of info on these trannny vacuum problems. If you are losing the brake power boost to, you either have a major vac leak or the vacuum pump has gone south on you.
This is all assuming that vacuum is your prob. I obviously can't tell you that it is. I've never had a vac system go bad and cause quite what you've described, but it is the easiest (and definitley the cheapest) thing to check before you got too involved. It could possibly be a B2 piston issue too. Research that here as well and compare to your problem.
I'm sure someone else will chim in and help you out as well.
Try the searches and see if that gets you headed in a better general direction.
Cheers,
-Matt-

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Currently:
1972 350SL Euro 4spd
1973 BMW R75/5
1981 BMW R80GS
1995 FZJ80 with OM606 Conversion In Progress