in a vehicle fuel system having a fuel tank with a filler pipe having an upper end closable by a removable cap and opening to the atmosphere when the cap is removed to receive an inserted object and a fuel vapor storage canister , a venting means that allows the cap to be removed while preventing pressurized fuel vapors generated in the tank when the filler pipe is closed from reaching the atmosphere . when the cap is removed , a valve is opened , while a seal means independent of the cap remains closed . thus , vapors are blocked from the atmosphere , but allowed to exit past the valve to the canister . in addition , the seal means engages a conventionally sized fuel nozzle , providing another seal so that fuel fill vapors are similarly controlled .