You should install 3 filters.

.1 Coming off the main line from the vacuum pump to the brake booster. Choose one of the lines that tap off the main line. This will inform you of a vacuum pump leak.Yes the vacuum pump is a diaphragm vacuum pump. The door vacuum pump reserviour which holds vacuum for the door locks, the check valve can leak and pull oil into the lines from the main vacuum pump.

2. The shutoff valve. the interior of the IP is bathed in oil. So if the diphram leaks oil it will get into the shut of lines.

3. The transmission line. The transmission modulator could leak and allow transmission fluid into the system. This one I have not done, but it is a potential fluid leak area.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car