In case the air compressor fails or a leak develops in the supply reservoir, a one-way check valve (Fig. 18) is installed between the supply and service reservoirs to keep air from bleeding back. The valve is spring loaded. Pressure at the inlet side overcomes spring pressure and lifts a check ball or disc off its seat. Air passes through the valve to the outlet. When pressure at the outlet becomes greater than at the inlet, together with spring pressure, the check device seals, preventing air from flowing back through the valve.