When I have confronted the situation that you describe, sun tortise, on my 617.952 engines...
1. Ready MityVac.
2. Clear mind of assumptions about anything.
3. ISOLATE and TEST *every* single vacuum sub-system that is connected to the engine shutoff circuit. What DO you mean? Within the engine compartment...
A. Disconnect the green line (climate control sub-system), connect the MityVac to it, pull vacuum, watch gauge for at least 30 seconds. Vacuum hold or drop?
B. Do the above for the solid yellow line (locking sub-system), yellow line with gray stripe (reservoir sub-system), blue line (seat back locks sub-system - coupe only).
4. As others have stated, pull the brown-with-blue-stripe line from the shutoff mechanism situated on the rear of the injection pump. Use the MityVac to draw vacuum on the mechanism. Vacuum hold or drop?

Now then, if the above still has not identified vacuum loss, there are certainly more things to do on a 1985 300D (that I have indeed done) to locate vacuum leaks. From this point forward, it just gets a bit more complicated.

If you do the above, what are the results?