An apparatus for regulating fluid pressure comprising an inlet for accepting fluid at high pressure and an outlet for supplying fluid at a predetermined pressure to the device that requires fluid flow. A piston and seal form the valve used to regulate the fluid pressure at the outlet. Another aspect of the invention is a high-pressure chamber to that provides a reserve of fluid for continuous regulated pressure during periods of sustained use. The regulated pressure is adjustable using a sphere and ramp mechanism. One embodiment of the invention is adapted for use with a paintball marker.