Quick release valves were developed to speed up the release of air from the brake chambers to let the brakes release quicker. There is typically a single supply port and two delivery ports. As the brake pedal is depressed and air flows toward the brake chamber, it passes through the quick release valve which is mounted nearby the chamber. The air pressure is held until the pedal is released. When the supply pressure at the quick release valve drops, the air in the brake chamber is released out the bottom of the quick release valve rather than having to travel all the way back to the relay valve's exhaust port.
Sealco 780219 Quick Release and Holding Valve with Moderate Flow Exhaust Seal
Bendix 229813N QR-1 Quick Release Valve, 3/8" NPT Supply, 1/4" NPT Delivery
PTP 229860 Quick Release Valve - 3/8" NPT Delivery Ports, 1/2" NPT Supply Port, 1 PSI Crack Pressure
PTP 288251 Quick Release Air Brake Valve - 3/8" NPT Ports, 1 PSI Crack Pressure
289714PTP Anti-Compounding Air Brake Quick Release Valve, Replaces Bendix 289714N