HVAC systems in the G-bodies were an exercise in stuffing 10 lbs of crap in a 5 lb sock. And somewhat complicated to understand what every component does in whatever mode you're in unless you've studied it for a while. When you go to cold, the blend door closes off the air flow to the heater core. On MAX, NORM, BI-LEVEL (any A/C mode) or DEF, the A/C system should be chilling the evaporator regardless of blend door position.
When the temp lever is in COLD it should also close the heater control valve on the engine. Anything off COLD will vent the 2-port valve in the control head to allow the heater control valve to open unless you're in MAX. MAX mode bypasses the 2-port valve and keeps the heater control valve closed all the time. (Hint- I'm talking about that hard-to-find-new 2-port vacuum valve on the back of the HVAC control unit)
In OFF, VENT or HEATER mode, the A/C compressor should be off. So in the diagram, air flows around the heater core area into the plenum. On hot, or somewhere in the middle, the blend door is partially or all the way closing off the bypass plenum, forcing some or all the air through the heater core to manually blend your temperature. If both are clean paths, you should not see much restriction in flow. If you're all the way hot and you suddenly don't have much flow, it's not the evaporator. And if your air flow gets cut down, your symptoms don't act like the blend door because it can't be in 2 places at the same time. The blend door is manual, so it works or it doesn't when you move the lever. Since you don't have flow when it's hot, then it seems to be closing off the "cold" flow bypass around the heater core. So this points me to something blocking the heater core itself. Which is odd, because the evaporator is always in front of it. It is quite possible the foam that's supposed to catch and drain water from the evaporator may have corroded away and the particles may have clogged your heater core. That's where the symptoms point out to me. Something is clogging your heater core path. While I will submit the heater core itself is a bit of a restriction, your decrease in flow should be minimal in an ideal situation.
Looking under the top of the HVAC housing under the hood, here's what you would see:
Evap obviously right after the fan, then the blend door is right after that. Look close and you can see the rod to the blend door coming through the firewall, and the foam water wick on the backside of the evaporator. Just look at the trash in there and it's a fairly clean considering the age of the original evap. Picture courtesy of
565bbchevy
The pic here is a generalized air flow from an F-body, but the heater/evaporator air flow is the same.
