The System Park Valve is a precision-engineered and manufactured product designed to provide optimum performance for each specified application. Its all-brass construction ensures years of dependable, trouble-free operation, and it comes as new construction, not remanufactured. Supplied with a mounting nut, this valve is used to control spring brakes on both tractor trailers, automatically applying when pressure drops below 35 PSI. With a stem diameter of .377", it is practical and reliable, fitting most makes and models. This valve is sold individually and is a crucial component for ensuring the safety and efficiency of your vehicle's braking system.