W123 Central Door Lock Question

[85chedeng300D]

Greetings Gentlemen! Here's the deal: On my '85 300D Turbo, the central door lock system will normally work when the engine is running but less than 30 minutes after the engine was shut off, the door lock actuator/diaphragms will not respond/react anymore on the driver's/main lock knob.

1. All 4 doors, including fuel door and trunk lid diaphragms holds vacuum and responds through a MityVac on both lock and unlock hoses.
2. The black vacuum reservoir in the trunk holds vacuum.
3. Transmission shifts normal and smooth.
4. ACC vent diaphragms works and functions as it should.

Now the question is, if anyone of you have had the same symptoms:
1. What could be the culprit?
2. Why is the central lock working when engine is running/idling and not when the engine is turned off.

Any help will be greatly appreciated! Thanks in advance and have a happy holidays!