Your problem is either a failed vacuum pump diaphragm or a failed shut off diaphragm. It's one of those failures that is allowing oil to get into the vacuum system. Many people put clear fuel filters in the main vacuum pump line and the brown shut off diaphragm line so that the presence of oil in the system can be easily detected.

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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