AC vacuum control head Q.

[Trackman101]

By design should the ac vacuum controller switch hold vacuum in the ac position even when the truck is shut off? Mine doesn't , I am trying to test to see if the switch is leaking.
My engine doesn't make a bunch of vacuum and when I am doing some spirited driving (ac on) my vac canister doesn't have enough supply to keep my heater valve closed. When the truck is idling or cruising I have enough vacuum to keep it closed. I have confirmed my canister holds vacuum, no leaks at the heater or door vent diaphragms.I have the 2nd vac port on the head plugged.