Thanks for all the help and insights!
Here's what I did:
1. Under the front passenger floor mat, I plugged/capped each line that goes to each door one at a time and run the engine until it starts to hold vacuum and operate the automatic feature but didn't find any one of them diaphragm that is leaking, but still would not hold vacuum when the engine is OFF.
2. Swapped the yellow check valve with a known good check valve, but still, the same, loses vacuum when engine is OFF.
3. Vacuum suction is between 20-22mmHg on both nipple whether the main vacuum hose is connected or not to the brake booster.

Now what????

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'85 300D Turbo - CA Version