Proportioning valves help meter how much brake fluid goes to the front and rear brake circuits, preventing rear brakes from locking prematurely. Fluid enters and exits via hard brake lines attached to the proportioning valve (here indicated by arrows). Drum brakes also require a pressure preload of about 10 pounds to keep the shoes in place and prevent excessive brake pedal pumping (to bring the shoes back in contact with the drums). When you switch to disc brakes you have to remove the parts that create this preload or your new disc brakes may drag. The fluid volume and pressure preload metering is done with floating pistons, sweeper seals, and springs.