When I owned an 84 300D for a brief while it struck me has having a very complex vacuum system....lots and lots of vacuum routing, which translates into lots of places for potential leaks. There are ways to simplify the vacuum system that can in turn make diagnosis simpler.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife