Gas-filled radiation detectors need gas filling at pressures that range from few cms of mercury to as high as 25kg/cm² at room temperature. Before gas-filling these detectors require evacuation to a vacuum of the order of ~1 x 10^-5 mbar. For these operations of evacuation and gas filling a system consisting of a vacuum pump with a high vacuum gauge, gas cylinder with a pressure gauge and a valve is used. The valve has to meet the three requirements of compatibility with high-pressure and high vacuum and high conductance. A piston valve suitable for the evacuation and gas filling of radiation detectors has been designed and fabricated to meet the above requirements. The stainless steel body (80mmx160mm overall dimensions) valve with a piston arrangement has a ½ inch inlet/outlet opening, neoprene/viton O-ring at piston face & diameter for sealing and a knob for opening and closing the valve. The piston movement mechanism is designed to have minimum wear of sealing O-rings. The valve has been hydrostatic pressure tested up to 75bars and has Helium leak rate of less than 9.6x10^-9 m bar ltr/sec in vacuum mode and 2x10^-7 mbar ltr/sec in pressure mode. As compared to a commercial diaphragm valve, which needed 3 hours to evacuate a 7litre chamber to 2.5x 10^-5 mbar, the new valve achieved vacuum 7.4x10^-6mbar in the same time under the same conditions.