The DeWalt 5140119-65 Air Compressor Check Valve controls the direction of compressed air between the pump and the air tank in compatible compressors. It is installed in the air line connecting the pump to the tank. The valve works by opening while the compressor is running, allowing compressed air to flow into the tank, and closing when the pump stops. This prevents stored air from flowing back toward the pump, maintaining tank pressure and supporting proper compressor cycling during operation.