Fuel Filler Cap

Fig. 15 Fuel filler cap:

The fuel filler cap, Fig. 15, uses a two-way (vacuum and pressure) relief valve that is closed during normal operating conditions. The relief valve is calibrated to open only when pressure in excess of .8 psi or vacuum in excess of .1 inch Hg develops in the fuel tank. When vacuum or pressure is relieved, the valve returns to its normally closed position.
Some filler caps have a rollover check valve. This valve consists of a stainless steel ball in a plastic housing mounted on the fuel tank side of the cap. Should the vehicle be overturned or tipped sufficiently, the steel ball drops into an orifice closing the vent to prevent fuel leakage. It is normal to occasionally encounter an air pressure release when removing the fuel filler cap.