In the course of changing the blower motor I decided to test the function of all the vent switch activators. Naturally all the floor and defroster valves worked like new. The one actuator that leaks appears to be inaccessible without removing the combination unit (entire heater box).

I can reach two fingers behind the box and force close the recirculate flap, and then it will hold a vacuum. Using a vacuum tester, if I release the vacuum slowly -- to the point where the actuator just opens the recirc door I can then pull it back in with vacuum and it will hold, but if I release all vacuum it will not again operate.

My hopes are, that someone has had this problem and can describe its likely cause, whether it is a chafed hose where it enters the actuator or (I hope not) a hole in the diaphragm (maybe hope Pablo will start a thread on that). Anyway, without help, I will most likely not fix this, as recirculate is the default position of this flap, I have no other reason to remove the heater core or evaporator, and I can always use the left side foot vent or open a window for fresh air.