In the diagram below, two reservoirs are shown. This gives the system a larger volume of main reservoir air. The first tank in the system collects most of the moisture, and is referred to as the wet tank. The second tank is referred to as the dry tank. Reservoirs vary in size. The size of the tank or tanks installed on a vehicle depends on the number and size of brake chambers.

To prevent air from flowing back from the reservoirs to the compressor, a ONE WAY CHECK VALVE (9) is installed. This valve allows the air to flow in one direction only. The valve is spring loaded. Pressure at the inlet side overcomes the spring pressure and lifts the check valve 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 the spring pressure, the check device seats, preventing air from flowing back through the valve.