A valve plug is biased inwardly by a spring (90) to close an opening (10) in a cap (2) for a fuel tank filling and venting tube. A valve stem includes a first portion (62) which is engaged by the spring (90) and a second float cage portion (40) that is secured to and extends axially inwardly from the plug. A collapsible retainer (70) is secured to the cage (40) and has an abutment surface (74) which engages an abutment surface (68) on the first portion (62) to hold the cage (40) and the first portion (62) in position relative to each other and to transmit axial forces therebetween. A fusible material ring (88) is positioned radially between the retainer (70) and the cage (40) to prevent the retainer (70) from collapsing. At elevated temperatures, the ring (88) melts to allow the retainer (70) to collapse, the cage (40) and first stem portion (62) to separate from each other, and the valve plug and cage (40) to move away from the opening (10).