Q&A

The heater a/c control head is a diverter for vacuum to be switched to different vavuum motors or servos,that inturn close and open air doors or flaps some may have 2 positions of opperation.If you hear a vacuum leak after you shut off the engine ,suspect a bad leak.If you can hear a leak with engine running its a worse leak or one at the control head.
You may even have a hose open or off the engine intake manifold from the main supply of vacuum for everything.If so the engine may idle a bit too fast and seem rough or run on or diesel when shutting off.Check under the hood first.You can check the small hoses from the control head and pinch them off to see if the air changes mode.There may be a check valve, in the vac.system and a vac.canister or reserv leaking as well. good luck.