Abstract: A closure for the filler neck on a vehicle gas tank having a closure body with a central passage and an upper surface having a vent groove formed therein. The vent groove has a first end communicating with the hollow portion of the closure body and a second end adjacent the periphery of the closure body. A cap is secured to the closure body and a resilient gasket is secured between the cap and the upper surface of the closure body such that a convex surface formed on the cap engages the upper surface of the gasket adjacent the vent groove in a closure body to deflect a portion of the gasket into the groove to seal the vent groove. The gasket is sufficiently resilient to permit passage of fluid through the vent groove when a pressure differential exceeding a predetermined magnitude exists between the first and second ends of the vent groove such that pressure within the tank is vented to the atmosphere.