Proheader PB202 - Residual Check Valve, 2 lbs BLUE, Disc brakes, each This is a Residual Brake Pressure Valve, 2 pounds, BLUE Anodized This is used for when the master cylinder sits under the floor making it lor than the calipers. Over time gravity and vibration will cause brake fluid to leak back to the master cylinder away from the calipers. When this occurs it causes a loss of brake pressure. With this valve it will prevent this bleed-back, acting as a one way valve eliminating return spring tension in the drums. Application info: A must have when running the master cylinder under the floor. Blue Anodized; Warning if you do not use this valve you may lose your brakes entirely or you will feel a drag. In short, this will help keep the pressure in the drums constant. A very necessary and inexpensive way to help out your brake system.