I can only speak of the 92/93 300D. They DO NOT run off the engine vacuum system. Their is a seperate pump under the back seat on the passengers side. It's wrapped in foam. (I'm installing a remote lock fob system 3/6).

Is the pump working? Did you check the fuse?
You can unplug each yellow line and see if you can get a vacuum.

Attached is the wiring diagram

Attached Thumbnails

 

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92 300D 2.5L OM602 OBK #59