First
Buy a mityvac so you can dignose each item on the vacuum system. Why replace everything if it's just one component.
Second. It sound like the vacuum pump diaphram has failed. Or just leaking. This is a common problem and requires rebuilding the vacuum pump. The rbuild is not a hard job.
Check all the vacuum connections, straigh,3way and 4way. Make sure they are giving you a good seal. Then with the mityvac check all the components in the vacuum system. Go here for the service manual.

http://skinnerbox.steaky.org/123_DISK2/program/123-cd-index.html

You are going to check the following systems, all vacuum related. Doors locks, engine shutoff, AC system actuators, EGR system, transmission shifting, brake booster, vacuum pump and injection pump. Isolate the interior systems one at a time to see if they are causing problems.
Clean all your vacuum lines with brake cleaner, allow to dry and reinstall them, Clean out the vacuum units that are contaminated with oil. rebuild the vacuum pump, or check to see if it is already done, and install inline filters to help you spot oil movement in the system.

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