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If you never get air out of the center vents, it's possible the vacuum pod diaphragm has failed. Or you may have other leaks in the ACC vacuum system, as the "default" position is air coming out the defrost vents (cold or hot).
If you can turn the heat off, your monovalve is probably OK. I'd suspect your pushbutton unit, as after 15+ years the 123 p/b units are failing more & more often - they get cranky at first, then get worse. The "aux" pump does NOTHING except keep hot coolant pushed through the heater core at idle - so forget looking at that unless you're complaining about lack of heat at long stop lights.
Is the A/C compressor turning on? If so, you probably have *some* charge in there. Wouldn't hurt to buy the R-134a recharge kit that includes a gauge, and pop that on the low side hose - the reading will tell you if your system is charged or not.
Good luck,
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