Abstract: A valve construction for high pressure gases, allowing rapid opening and closing, includes a main body with inlet and outlet passages, and a guide sleeve having a primary and a secondary piston slidably mounted within the sleeve, the primary piston being adjacent a closure at one end of the sleeve, and the secondary piston being at the end of the primary piston which is remote from the closure. The secondary piston supports a seal for closing the gas outlet passage when the secondary piston is remote from the closure. The secondary piston has an axial passage with an elongate member movable therein, the elongate member being fixed to the primary piston. A valve means is provided between the axial passage and the elongate member, the valve means being open when the bleed orifice is closed. A peripheral seal between the secondary piston and the sleeve acts to restrain the axial movement of gas.