When you measured the vacuum out was that a direct connection to just a vacuum gage or did you tee into the tube going to the transmission? If the later, it could be that the modulator on the tranny is leaking.

My 1985 CA 300D has the blue saucer, which I assume is the same PN as 1986+ cars. The VCV in my car is progressive, and the same PN as earlier cars. I put one on my 1984 300D, plus the 85 CA frame-mounted air cleaner. I see them often in CA junkyards and inexpensive. I may be going this Fri (1/2 off day) and recall a 1985 car, so might pull it to have a spare. If you get desperate PM me, but should be plenty there from 86+ cars. If limiting yourself to new M-B parts, your car will soon bankrupt you. Good luck.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans