This is not right. The pump should stop when all the plugs are up/down. If it doesn't, then you'll got a leak somewhere. How long can you hear the noise of the pump before the doors are unlocked?
I have also heard about broken membrans and that could maybe cause something similar.
You could trace the leak done by systematicly follow the vacum hose and pull one at a time and plug the hose, then try to lock the door. . You'll know where your leak is when the pump cuts out immidatly when the three remaining doors are lock.
I can describe it more specific if you want to.
The average time for the pump to open/lock system should only take about 2-3 seconds. Not more...
'87 MBenz 190E 2,3 8vlv