Abstract: A shut-off valve for use with high pressure gas cylinders is disclosed which is adapted to automatically retain a residual pressure in the cylinder. The valve includes a check plug axially moveable within the housing which has a hard sealing surface, for establishing high pressure seals and a soft sealing surface for establishing low pressure seals. The plug is spring-biased into the closed position with force selected so that it automatically closes and seals the interior of the cylinder once the gas pressure in the cylinder drops to or below a predetermined threshold pressure.