the fuel filling system includes double pipe fuel delivery and tank venting plumbing . both fuel delivery pipe and tank venting pipe are flexible and communicate with the gas tank through normally closed check valves . an inlet housing defines a vent cut - off chamber which communicates with the tank venting pipe , and also includes an annular nozzle embracing seal for isolating the end of the gas pump nozzle from the vent cut - off chamber . the gas tank is thus fully sealed by the check valves during driving , so that fuel will not spill from the tank should the filling system plumbing become ruptured or should the vehicle become overturned as a result of a collision . fuel is introduced into the tank under positive pressure provided by the filling station fuel pump . under pumping pressure , the check valves opens to permit the inflow of fuel and the outflow of air displaced from within the tank .