The master valve in the driver's door is probably the last part to wear out, although it does happen.
Borrow your friend's hand pump and start at the check valve. It will be in the line coming off the manifold, before the "Y" and should be blue. Disconnect the vacuum line leading to the "Y" and pull a vacuum on the valve. Does it hold vacuum for 15 + minutes? If not replace it (about $10.00).
Next, disconnect the leg at the "Y" that goes into the left fender. That will remove the reservoir from the equation. Be sure to plug that side of the "Y". The reservoir is well protected and is most likely not the cause (mine still looked brand spanking new when I pulled the fender liner to check it). Taking the reservoir out of the mix will allow any vacuum leaks to be readily apparent.
There is a way to cut and splice the vacuum lines to test the individual actuators, but I prefer to pull the door skins and check each actuator separately. I imagine you have at least two torn diaphrams (there are only three actuators for the doors, one in each rf/lr and rr doors). You probably have a bunch of decomposing connectors as well and now would be a good time to replace them using appropriately sized rubber vacuum line.
If you find that two diaphrams are bad, I recommend replacing all of them. This was recommended to me and I didn't listen. Sure enough in 6 months I had to do the last one.
The 108 acutators are rebuildable if you can locate the rubbers. If you can't find just the rubbers you can replace the entire unit. New units usually come with the push rods attached.
Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.