air switches from vent to defrost

There are several vacuum 'motors' in your A/C vent system. it sounds like the vent activating diaphragm is leaking. I would also check the vacuum line under the hood that goes to the A/C system.

you may have a vacumn leak or a vacumn canister in the system that leaks,this is used to hold the vacumn settings in the control head as you accelerate or go up hil in wide open throttle.The only check valve i know of is the one conecting the vacumn booster for the brakes ,usually the one intering the booster body.from an intake manifold hookup.