A self-sealing valve for controlling the volume of fluid within an inflatable device. The valve includes a housing having a fluid conduit, a valve seat and a valve assembly that selectively seals the fluid conduit to provide a self-seal. The valve assembly includes a support member and a mounting device that holds a portion of a flexible diaphragm in a closed position, and that facilitates movement of at least part of the periphery of the diaphragm away from the valve seat to an open position and towards the valve seat to the closed position. The support member may be manually moved to any of the multiple open positions, either to enable fluid release or to enhance fluid injection. With the injection of fluid, the diaphragm assumes an open position, allowing fluid influx.