It is probably due to air ENTERING the system presurized by the Fan Motor for the bag.
Rather than PLUG to prevent, rather than PUT A HOSE to vent......why not close the source?
Air Temp valve in air cleaner, charcoal canister hoses or air cleaner filter vent.
Not sure what car this is but depending on that pretty easy to FIND the source working back from the fuel vent line on the tank.
Mark