OK. That I get.
I remember reading somewhere that oil would get into the vacuum lines by a failure in a vacuum pump diaphragm and the clear filter would give warning of a failed pump diaphragm. That's the part that doesn't make sense. Seems to me that oil in the vacuum system must be coming from the shut off diaphragm and not a failed vacuum pump diaphragm.

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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