Just came back inside from draining the system again. Hit the radiator and reservoir with the hose.

Also pulled the valve on the inlet side of the heater core. I tested for vacuum and it is fine. However, I did notice that the leg which moves the valve inside was not connected. I don't know if that was something that was preexisting or something that I may have done inadvertently when removing it (although I have a hard time believing I did).

Given the vacuum tube was plugged AND this was previously a Florida car with a known A/C issue, my guess is that it was pre-existing. Particularly if the HVAC system is set to expel heat when things break.

It's raining now, so I'll head back out later to flush out the heater core and put things back together.