99 Jimmy climate control problem

When I have my climate control set on auto fan and floor vent or mix and put the engine under load (passing on a highway or climbing a steep upgrade ect.) I get outside air through the panel vents. This happens no matter what setting the vent selector is in except panel even if the heater is off. Many trips to the dealer finds nothing. They replaced a vaccum canister they said leaks as well as hoses and even the control panel but it still happens. It sounds like a simple check valve problem to me. One other symptom is that if I shut the car off for 2 min or more when I start it back up I get air in the face till the vaccum lines charge. Anyone ever have this problem and how do you fix it.

Sure sounds like a vacuum leak to me. can you plug the lines going to the actuators that open the doors and see if it still leaks down after that ? If so then an actuator is leaking through.
Your just going to have to try and isolate each piece until you find the critter.Good luck

Could also be a leak anywhere else in the vacuum system, not just the heater controls stuff.
Maybe power brake booster ?
Get a vacuum gauge and see if the vacuum drops off when the gas is revved up, as the high vacuum is needed to keep the doors closed, not to open them.If the vacuum falls when accelerating it probably goes below whats required to keep the vent doors shut.