Heater Vacuum lines issue?

[VAPhil]

Happy Tuesday everyone.

Got an issue I haven't been able to solve. My heater in my 1972 Chevy C-10 only works if I have the heat levers engaged when I start the engine. If I try to engage the heater while driving, I only get cool air. Heater works fine (really hot) otherwise.

I get a "swishing" vacuum leaking sound coming from the 2-nippled vacuum switch on the back of the heater control unit. Sounds like the plates aren't sealing. Could this vacuum leak be causing the problem? Unrelated? Thoughts?

-VAPhil