A single check valve is installed at the inlet of the air reservoir that is fed by the air compressor. This valve prevents the loss of service brake air pressure if a leak occurs in the supply portion of the system. The single check valve has a supply port and a delivery port. Normal air flow unseats a disc inside the valve and allows air to flow from the supply to the delivery port. A pause in the air flow or flow in the reverse direction causes the valve disc to seat, blocking the reverse air flow.