A two-way check valve allows compressed air from two different sources to flow through the same air line. The valve will select the source with the higher pressure. In this case, a two-way check valve is connected to both the primary and secondary service tanks to always draw air from the tank with the higher pressure and send it to the parking brake control valve. This arrangement will ensure that the higher pressure is available to release the spring parking brakes. It will also ensure that the spring parking brakes will not activate if pressure is lost in just one of the two tanks.