Why does my 92 Ford Chateau club wagon AC blows through the defroster vents?

under hood on driver side is a check valve,round plastic part the vac lines hook to is bad,can get at any auto parts store in the help section.

Thanks, couldn't find any check valve under hood on Driver's side. I checked all hoses from the intake manifold (source) to the vacuum reservoir and found no check valves nor leaks in the system. I added a check valve to the vacuum actuator for the a/c / defroster door. It seemed to help some but I still loose vacuum when accelerating even with the check valve added. I have seen technicians add smoke to the intake system to check for leaks. Where might I find this "smoke"?

is at firewall at break booster,close to inner finderwell.is 3 vac lines hooked to it.