A valve that charges the trailer reservoir(s) with air, applies & releases the parking brakes and applies the emergency brakes if a catastrophic loss of supply line pressure occurs. Determines the priority of air delivery when supply line air is applied based upon the design of the SBCV (see Protected Reservoir System, Spring Brake Priority & Service Reservoir Priority). Used on post FMVSS 121 trailers with spring brake chambers, SBCVs combine a quick release valve (QRV), pressure protection valve (PPV) and, in all Sealco SBCVs, a one-way check valve (CV) for anti-compounding.