A fuel cap for closing a threaded filler neck of a fuel tank is disclosed. The can has a central button or driver which reciprocates within an opening in the crown of the cap. When the cap is installed on a filler neck, the driver is in an upper position and the crown freely rotates relative to the threaded housing in a cap-removal direction. When the driver is depressed, a pressure relief valve is opened and a driving relationship is created between the crown and the threaded housing in a cap-removal direction, permitting the cap to be removed from the filler neck. When the crown is rotated in a cap-installation direction, the driver is moved to the upper position and the pressure relief valve is closed.