A flip-top replacement gas cap for replacing conventional screw-on caps provided as original equipment with present day motor vehicles. Two embodiments of the invention are disclosed. The first is a manually actuated device. The second is a key actuated device. In both embodiments pressure relief valves are included to vent over-pressure within the gas tank and to bleed off pressure prior to opening of the flip-top cap. The key actuated embodiment includes a rachet cam assembly which permits the cap to be screw fastened to a motor vehicle gas filler vent but precludes the unscrewing of the cap thereafter. "Gas cap" is used herein as a generic descriptor for any closure cap used to seal a filler tube of a fuel tank or any other storage vessel.