Start by getting a mityvac and determining which subsystem is at fault. For example you can isolate the door locks and the vacuum reservoir at the firewall. Chances are good that the door locks are the culprit. Get familiar with the vacuum diagram for your car for help in tracking the problem down (find which components don't hold vacuum which should).

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For Sale: 1982 MB 300TD
1995 Chevrolet Suburban 6.5TD

Sold: 1980 IH Scout Traveler- Nissan SD33T Diesel