I just bought a 1980 300SD. The front windows and locks dont work all that well, and the back windows and locks dont work at all. Advice?

Welcome, new owner.

The locks, and a lot of other equipment, are vacuum operated.

A vacuum leak, or leaks, will slow or stop operation.

A lot of posts have described troubleshooting. (There is a handy search function, 6th from the left, blue bar above.)

You'll want a MityVac, which is a vacuum source and gauge in one. Vacuum diagram are posted in this forum, 2nd 'sticky' from the top. You'll need a heaping helping of patience, and some spare time.

You will isolate the locks from the rest of the car and test to see if they hold vacuum. If so, the leak is elsewhere.

It's going to be a game of hide-n-seek, and the leak has already hidden.

Hope that gets you started. We're all here to help, do some research, and let us know how we can help. We'll want specifics about what you've observed, what you've done, how much experience you have working on cars, etc. to be able to help more quickly and accurately.

Best Regards,
Jim


P.S. Come join us in "Diesel Discussion"
Jim