66 Cuda HVAC control problems

Double check that vacuum servo. Some of them have vacuum both ways instead of vacuum one way spring return the other. Could be a failed return spring.
When a servo has vacuum both ways ( like the one at the water valve ) the vacuum that was applied one way needs to be vented whenever vacuum is applied the other way. If you had a unused port on the switch and thought it best to cap it... you probably blocked the vent.
I'm not sure when they started adding the porous stone that served as a muffler for the vent.