I have a 1980 300D and i rebuilt the vac pump several times. The first time repaired the ripped diaphragm and I replaced the valves as well. The new valves failed < 10k miles later. Replaced those valves and the new ones also failed < 10k miles later. SEveral shop mechanix told me they never bothered with rebuilding vac pumps for this reason. I couldn't figure out the failure mode for the valves, but I put a used pump in I pulled out of a boneyard and it's been good for > 30k miles now.
I don't know what to tell you, other than give it a try and beware if the valves keep failing.