The Four Circuit Protection Valve 3515PF for Dongfeng is a key safety part in heavy-duty truck air brake systems. It splits and sends clean air from the compressor to four separate lines (front brakes, rear brakes, parking brake, and extra tools). If one line breaks or leaks, the valve quickly seals that broken line. This saves enough air pressure in the other lines so the driver can still stop the truck safely. [1, 2, 3, 4, 5, 6, 7]