From your question I take it that this leak occurs when you lock the door, and that you have some reason to think that the act of locking the door is what causes the vacuum loss.

Since the driver's door lock actuates all the other locks, the leak could be anywhere in the locking system.

Are you sure the system has vacuum before you try and lock the door?

__________________
Ralph

1985 300D Turbo, CA model
248,650 miles and counting...