In OEM vehicles, they are present in drum brake systems. 10 lbs of line pressure are required to keep the wheel cylinder cups expanded and ready to start moving the brake shoes. They were either located within the master cylinder behind the brass inserts at the exit ports, or down in the combination valve. Without them, you would experience long pedal travel. For cars with disc and drum brakes, they would only be used on the drum side of the system. If you had four wheel drums and replacing only one side with disc brakes, the drum side will still need the 10 lbs of pressure.
In vehicles changing to all disc brakes, it becomes very important to get rid of the 10 lbs valve if the vehicle previously had drum brakes. The constant line pressure would cause the disc brakes to drag and overheat.
It is common in a Drag Race vehicle to mount a master on the floor or frame rail. Since it’s below the level of the calipers, the brake fluid wants to return to the master cylinder due to gravity. Assuming there are no leaks, the pistons retract back into their bores. When the brake pedal is applied, an extra volume of fluid is required to move the pistons back out to contact the pads and begin applying pressure. This creates long pedal travel requiring the use of 2 lbs residual pressure valves to offset this condition. We do offer external 2 lbs valves (B3366) and a 10 lbs valves (B3367) to suit your needs.