How it worksThe wheel cylinder consists of a cylinder that has two pistons, one on each side. On the interior of the piston is a rubber seal. The piston makes contact with the shoe directly or through a slotted pin. When hydraulic pressure is applied by the master cylinder, the cups and pistons are forced out, pushing the shoes into contact with the drum. When the brake pedal is released, return springs pull the shoes back away from the drums and push the wheel cylinder pistons back into their bores.