This is really a job for a pump shop, but the adjustments are exact and undoable (not particularly easy though).
I would suggest replacing the pneumatic governor diaphram noting the shims. Don't change any till you have tried it with the new diaphram (that could be the problem). You will see that vacuum pulls the pump rack one way and the spring the other. Changing the shims changes this relationship in small quantities.
It would be trial and error and once you are good at it it will only take 15-30 minutes. The first time you do it you might swear it can't be done on the car; it definitely is.
Bosch Master, ASE Master, L1
33 years MB technician