Sam,
I had an '80 300TD (wagon), and I can tell you one thing. 1980 is a very inconsistent year. There were more than a couple of changes "cut in" in 1980, which is why it is often described as a "transitional year" for these cars. The vacuum systems, the change from climate control II to climate control III are a couple of the examples.
I would recommend you dig into the search function of the archives and read posts and peruse some of the pictures. Learn which line does what, where it goes, etc... and you can solve the mystery of your vac issues.
One thing to note: into the firewall, next to the brake booster, some of the vacuum lines go though a rubber thing. Vac lines are color coded thusly:
yellow----door locks
yellow/gray-----vac reservoir for door locks
Green/yellow-----climate control vacuum (if you have climate control II this might not apply, I'm not sure (climate II has vert. buttons, climate III has horizontal)
Brown-----key switch for shutoff vacuum
black-----just a vent line that hangs under dash
You can isolate one circuit at a time, if need be, for trouble shooting purposes. A golf tee and some extra vac line will help.
Keep us informed of your progress, and any additional help we may offer.
Welcome
**edit**
The post below has the diagram that matched this wagon.