Do your searches as Kerry advised, then come back with your specific questions. If you have fluid, and it doesn't leak out and appears to pump, suspect your accumulators.
"It always the accumulators". [also often called spheres or air cells even though they're nitrogen]
**edit**
If you look at top of page, you see a "Buy Parts" icon. That is Fastlane, our fine forum sponsor. Whenever possible we try to utilize them, as they provide this excellent forum space. If you need accumulators, call Phil the parts guy, and see if he can do better on the price. Most other places are below a hunnert bucks apiece for accumulators......
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1VK01GHCF208027Y3F&year=1981&make=MB&model=300-TDT-001&category=L&part=Accumulator