Air Flo Switch on

check small vacuum hose on rear of square inlet manifold top that goes throught firewall ( bulkhead ) is not cracked or has come off
it goes into cab near heater hoses. this is the vacuum supply hose
if you have no vacuum the heater controls default to go to windscreen. also check the black & white plastic little round one way valve for leaks & operation
also check vacuum hose that goes onto heater tap that is in right inner gaurd under brake master cylinder area for leaks and that its on the heater tap valve
if they are all ok check inside do you hear any hissing noises behind dash controls or down at front centre console area?

Couple of possibilities here. One is the heater core bypass valve that allows the coolant to flow to the heater core. Make sure that the vacuum tube has not come off of it. Should be under the hood just on top of the blower motor assembly. Let the engine get hot and check to see if the hoses going to the heater core are getting hot coolant flowing through them.

Does the air flow through the defroster, front vents, and floor vents when they are selected? If not, this can indicate a vacuum leak on the HVAC control panel or a failure of the switch.

I would check under the dash to see if a hose has come disconnected for the middle vents

Mine does the same. Sounds like you checked the air pressure switch, but the high temp switch and bad airflow can also cause this problem. Check the two white wires that lead to the heat sensor vent drain tube exits. Make sure the spade connectors aren't shorted. Try testing the temp sensor like you did the pressure switch to make sure the circuit is properly closing when in use (or use a temporary jumper to bypass as a test.) Check the bottom of the pan for any dust or crud that could block the airflow below or around the burners and last of my options- increase clearance around the unit to aid air flow.

You have lost engine vacuum to the AC vacuum motors, this is either a pinched vacuum line from the engine vacuum storage canister or the canister is damaged or the control switch in the dash is leaking vacuum.

you have a loss of vacuum. There is a small diameter black plastic tube from the engine compartment that feeds the climate controls. Check at back of control head, or there is a plastic connector under dash were other color tubes join to operate the other actuators of the system the black tube must have vacuum when engine is running. check under hood the black tube is spliced. separate the tube and check for vacuum The tube enters the firewall near the heater hoses.

Check your vacuum lines under the hood and particularly the vacuum reservoir which is a canister that supplies a reserve on vacuum to bridge the times the engine is pulling hard. These often have plastic nipples with pushed-on hoses that can snap off.
The directional control of air flow is most often accomplished with vacuum 'motors' that move vanes to change direction. 

The vacuum line on the drivers side of the intake manifold just below the throttle body fell off.