Don't rebuild or replace the pump until you're sure that's your problem. With every thing vacuam operated on the car failing as it is, it may be your problem. To check it, you can disconnect the connector at the pump, connect a vacuam guage and start it up.
If you have vacuam at the pump, then start moving upstream until you find the leak.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in