Thanks for the reply - very helpful.

I've checked the temperature sensor and found that the wires are still solidly connected to it. Incidentally, i recently replaced the rubber tube that connects the sensor to something under the dash. The spongy section of the old tube had disintegrated, so a new one was in order. Not sure if the replacement has something to do with my problem.

This morning, after manually opening the center vents, I was able to get some moderately cool air coming thru the vents. It was enough to keep the cabin comfortable in the 100 degree phoenix heat. But later in the afternoon, the outside temperature rose, and then the air was blowing hot - very hot. I checked the compressor and found that it wasn't engaging. So now, instead of jumping to get a replacment, i'm wondering if any of the vacuum problems could be the cause of the compressor not engaging. I may be low on freon as well, so that could cause this problem.

As for 40, which apparently has something to do with the broken heater box (it broke right at the two rivets), the only way to check or service it is to completely dismantle the whole dash? When I manually move the center vents, they don't bog down at all - they move perfectly smoothly. This should indicate that things inside the heater box are ok, right?

And finally, once the car is turned off, there's no vacuum left in the system, even after a quick restart. This leads me to believe that there's obviously a leak somewhere.

Thanks again for the help. Slowly, but surely, i'm going to figure this thing out and fix it!