Almost certainly a minor problem. Could be a leak, disconnected line, bad shut off diaphragm or bad vacuum pump. Non of these are serious problems especially if you have any shade tree mechanic experience.
These kinds of problems are godsends when when purchasing an MB diesel because they keep the buying pool small. It does say something about the mechanical ability and marketing intelligence of the seller.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13