I'm thinking of buying a 300D. It starts ok, but doesn't shift properly (automatic transmission) and won't turn off with the key. The guy who owns it says that it is the vacuum pressure pump. He had rebuilt it previously. Anyone know if these symptoms are from that? How can I check it? Also this guy can't find the information to rebuild the pump again. Where do I find that info? I have a haynes and chiltons manual but they don't go into it. How hard is it to do? special tools, parts, etc? I have experience working on cars. Thanks for any info you can give me.

 

Rebuilding the vacuum pump means replacing the membrane. On Bosch pump it is not a brainier. about 8 screws (Allen head if I remember correctly) put the new membrane and screw it back.
Lot of the problems on those cars come from disconnected vacuum lines, so if you are lucky the pump might be still OK. Check the vacuum lines, especially around oil filter.

 

AFIAK the most popular kind of vacuum pomps in MB was Pierburg. There are two types: with a membrane (common in W123) and with a piston (W124, W201, W140). You're right, in case of a membrane pump replacing of the membrane usually solves the problem. In the latter type the weakest link are separating valves, unable to repair.

IMO there is no point in rebuilding the vaccum. New, genuine Pierburg costs in Poland about 250$ and this would be probably the last pump in the car's life.

The best way to know whether your pump is ok or not is finding some good equipped garage, where you can have your pump checked. A good one generates a vacuum of 0.75-0.8 bar. But before you do it check all the vaccum lines.