I discovered why the pod inside the heater core wasn't opening the door to which it's connected despite holding a vacuum. Turns out, the rank amateur who rebuilt it forgot to reinstall the spring. It now opens when I turn the temp dial all the way to cold.

Nevertheless, I still haven't seen the pod at the top of the heater box operate via the control panel. It holds vacuum all the way down to the manifold.

Corvo Gold
1987 300TD, converted to manual shift