I know it is a vacuum problem, but my problem is where. (I'm not trying to be a smart-A, even though I'm told I am very good at it.)
I have a vacuum pump, have worked through everything that is connected, (the door locks are disconnected and plugged), and it is pretty tight. The HVAC sysem seems tight, but I have not checked all the valves individually. I can pull 15+ in Hg when the motor is off and transmission line disconnected and plugged, and the vacuum will hold steady. With the transmission connected, I'm don't get a total seal, but the only time the guage drops to zero (like a rock) is when I turn the ignition switch on or off. So I know I have a small leak in the line to the transmission valve - at the valve itself. When connected, it takes a longer time for the vacuum to go to zero inches, and the rest of the system does hold.)
This problem started suddenly after coming home in the rain. I don't see how the rain could cause this problem - but stranger things have happened. (Maybe it washed out the wrong piece of dirt!) I have ~20 in Hg at the brake booster and ~10 in Hg of the tee to the rest of the system. The vacuum in the ignition switch will hold when the engine is running, OR when it has stopped. But vacuum will suddenly go to zero inches Hg when the key is turned. I tried hooking the vac-pump to the inlet side of the ignition switch and starting the engine, then turn the keyoff and removing the key. The engine continued to run till I reached about 8 in Hg vacuum which stopped the engine. That's why I think it is the switch and the act of turning the key causes an opening to the outside air (if it was electrical I would call it a direct short to ground). BTW, I think the electric part of the switch is fine.

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