1993 Ford Aerostar Air control valve to dash vents

Your problem is the vacuum feeder line that comes in from the vacuum reservoir in the engine compartment. either the line is broken, or the reserve tank is cracked.

Guys, I didn't see anything wrong with the vacuum reservoir (above the evap housing in the engine compartment) or the lines I could find. I held the vac tank with the fitting down in the swimming pool for several minutes- if it had a leak I should see bubbles as air came out and water came in - no sign of a leak and no cracks evident. I later put a short piece of hose on it, sucked it out by lung power and clamped off the hose. About 10 seconds later I loosened the clamp and heard air rush back in. The tank is okay.
I also loosened the A/C control panel on the dash with the switch that controls all of this - Vent, Norm A/C, Max A/C, Floor, etc. It has many vacuum hoses, but I did not see any obvious crack or bad hoses.
The line from the vac tank enters the heater/blower box under the hood- I tried to remove the blower motor and fan to see inside - wher it went- but couldn't get it out with the A/C accumulator in the way. On the inside, I removed most of the dash and instrument panels, looking for more vacuum motors. I found one under the right-side top panel (sort of above and forward of the glove box) that seems to be involved with air distribution under the dash. I removed it and I could suck on the fitting and actuate it. If I manually push it to the actuated position and block the vacuum fitting, it does not leak and return- I think it's okay.
Is there another vacuum motor under the dash,
somewhere else, that is causing this change in the airflow output? Joel

On the feed line look for the small check valve, it will be on the line from the manifold, if the check valve allows suction both ways, replace it. You are looking for the white vac line and the check valve, if it works OK check the vacuum line to the auxillairy heater in back.

Test the vacuum at the control head, if it's as shown, the leak is in the enine compartment, or the rear line. You may have a vacuum leak, get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak,