Brian,
Your prompt [ literally overnight for me ] response to my plea regarding a vacuum diagram… well it harkens me back to one of the many required memorizations I had to learn as a Plebe at the academy:
“ No soon said than done… so acts a man of worth ” ! And I had to learn that little “diddie” in 1964 and up it popped from a lot of cob webs !

However this 1980 300D does have an EGR valve ! I think these 1980 models M-B(s) were subjected to some kind of internal purge within the R&D group at M-B and this explains why I’m having such a hard time coming up with more certainty in this and other searches for answers. I also have a 1980 240D which had a dealer install engine with only ~13K miles on it... and does not have an EGR valve.

Do you or anyone else know if one can expect results if I were to go to the local parts department of my local M-B dealer armed with the VIN #... would this likely result in my obtain an " official ” vacuum diagram? I suspect that your's is probably close Brian... and I have others that show how the EGR is connected so I think I'm on the right path now. I have a new vacuum tester coming soon so hopefully I'll be able to sort out this vacuum thing here soon.

Brian, your response has made me feel like I have arrived at the inner sanctum of this DIY's forum… thanks,

Sam Ross