Probably has nothing to do with the actual vacuum pressure in the system then, just coincidence that it opens under high vaccum. You should check the electrical connection between the switch and the vacuum control unit. I think the control unit for that is just inside the area where the glove box comes out, and attached to the wall on the left. I'll check the factory service guide for you when I get home. Anyway there is an electrical connection from the dash switch that controls when that vac solenoid opens. You should check for voltage to that solenoid. There are two there I just can't remember wich one does what. When the switch is not working or the solenoid leaks it does weird things with the mixing flaps. and the fresh air door.
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