Vacuum Leak: 81 300D

I have an intermittent vacuum leak and suspect something in the system involved with transmission vacuum regulation. The leak manifests itself as slow engine shut-off and weak door lock operation--both these systems are solid (replaced elements recently)and work fine if I disconnect and plug the line going to the assembly on top of the valve cover. The vacuum modulator on the transmission holds steady vacuum when tested. I know there are three 3-way switches under the plastic cover and presume it could be one of these? Possibly could be the white plastic mechanism on the back of the injection pump? Thanks for any suggestions.