It could be the check valve from the engine to the resovoir, it could be a leaking hose. There is a $50 handheld tool called a "mighty vac" it allows you to draw a vacuum, and a guage to see if it holds it. The system as I recall from my old 85, was a valve in the drivers door, operated by a rod from the key lock, which routed vacuum from the vacuum tank to the diaphrams to lock or unlock the doors, trunk, and gas door. The vacuum is derived from the engines vacuum pump thru a check valve.