The vac pump for the locking system is in the trunk, spare tire well, I think. Has a single hose, provides vac to lock the doors and pressure to unlock.
You can test the locking system with a mitivac hand pump -- vacuum should lock all doors, gas filler, and trunk. Pressure should unlock everything.
The pump must run and provide vac when the driver's door, passenger front door, or trunk is locked, and also provide pressure when any of those three are unlocked.
Check the electrical connection -- it can get corroded and no provide enough juice to the pump.
Also check the condition of the rubber connector for the vac/pressure line -- if it is in poor condition, insufficient vac or pressure will be delivered to the system.
If the pump is OK, then you will need to find the leak -- either you have a bad rubber hose connector or a bad actuator. The door actuators are in the bottom of the doors, the trunk one is on the right side of the lock, under a cover panel, and the filler door one is on the upper right side of the trunk, behind a panel.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!