Sounds like the pump is working as it should, and one of the door actuators has a leak. The pump keeps running, the leak won't seal and the pump times out and shuts down. You need to check the condition of the door actuators and all the vacuum line connectors.
'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.