You most likely have a leaking lock diaphragm at one of the other three doors, the fuel tank door or the trunk.

Start by opening the trunk and disconnecting the lines to the fuel tank door diaphragm and hold your fingers over them while you have someone lock the drivers door with engine running. If the other doors lock the problem is the fuel tank or trunk diaphragm, if they don't it is one of the door diaphragms. Just continue to check the vacuum as it goes down the line until you find the leaking culprit.

Good luck,