Heater/AC Unit Diagnostics

[53Rick]

Hopefully there is someone out there who can walk me through the operation of a heater/AC unit. I have a '72 GMC 1/2-ton with a 402 and factory air. The heater controls are all connected and appear to be operating as they should. The AC will work, but the problem is in the heat side of the equation. During normal operation and with everything off for the heater/AC, I get low volume hot air through the passenger side dash vent and also through the AC Center vent. In fact, the chrome center vent gets too hot to touch at times. With the unit turned to defrost, I get low, low to no air flowing to the defrosters. The duct work appears to be fine and the duct work to the defrosters was replaced just a couple of years ago. The AC heater valve that mounts under the hood has been replaced. I have a shop manual and have tried to make heads or tails out of the description and the diagrams, but there is no troubleshooting guide. I'm not sure what would be the most likely culprit. I could start replacing actuators, relays, etc., but would rather get an idea as to what most likely is the cause of the problem. It almost seems that there is a door inside the unit that is failing to open or close as it should. The AC was not working for a number of years, but I was able to get that fixed although the AC volume isn't all that good. Any help would be appreciated. By the way, the truck is currently in a paint shop getting a new paint job and will stay the factory green and white 2-tone. I'll post some pics when it is finished.