I have a '77 240D which will not maintain vacuum when doors are locked. If doors are not locked, vacuum is maintained without a problem.

So far, I have:
-Replaced vacuum pump diaphram. I pumps @22" of vacuum
-Checked blue and black check valves under the hood, they hold.
-Replaced Master Vacuum Valve, Driver Door.
-Checked diaphrams in all three other doors, discovered no leaks.
-Put all night test on vacuum tank-it's good.
-Tested gas flap and trunk diaphrams-they hold as well.
-Replaced 3-way and 4-way vacuum fittings under the hood.

I have been to my usual mechanic, who can not figure it out. The local dealer was also no help.

What am I missing? I think there is a checkvalve I am missing. I need help, if I can diagnose it, I can fix it.