another dumb lock question
 

I searched and got some good info but am I right to assume that when I lock my driver door with key that all locks should lock?
81 240d

It has worked so far but no I'm having trouble now with the passenger rear door.. I lock it and it gradually comes back up. I checked for leaks and found none.

You are correct.

In normal operation, the unlock side has vacuum applied whenever the doors are unlocked and the lock side is vented. When the doors are locked, vacuum is applied to the lock side of the diaphragm and the unlock side is vented. The locks should stay locked even if the vacuum is lost.

This sounds like there is a vacuum in the unlock side of the diaphragm (not vented as should happen) and you are losing vacuum on the lock side.
Have you confirmed that the vacuum does not leak in the locked position?
Easiest to access the vacuum lines under RHS front carpet, along the door sill.
Check for kinks in the vacuum lines to the errant door actuator.