It was probably disconnected and plugged due to a leak in one of the elements. You need to get a Mityvac and test your elements. More than likely the unit that was disconnected, is the problem. Use the Mityvac to pull a vacuum on each side of the diaphram. Attach the Mityvac, pump it and you should, 1. be able to hold a vacuum, and 2. the lever coming out of the element should move. Vacuum to the other side of the diaphram and the opposite whould happen.
If one side will not hold a vacuum the element needs to be replaced. You cannot rebuild the unit, they are about $25.

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