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, the foot valve and hand valve are connected to two-way check valves that in turn lead to the tractor protection valve, and from there to the control (service) line. For example, if the foot valve is sending compressed air at 30 psi (207 kPa) and the hand valve is sending compressed air at 40 psi (276 kPa), then compressed air at 40 psi (276 kPa) will be sent out to the trailer brakes.