Residual pressure valves are a one-way check valve used to keep a small amount of pressure in your brake lines, helping the brakes engage more quickly and reduces pedal travel. The 2 PSI version is usually used when the master cylinder is lower than the brake calipers. The 10 PSI version is to be used with drum brakes which helps to hold pressure along the return springs. 1/8” NPT & comes with 2 x 3/8”-24 nuts.