This brake cylinder converts hydraulic pressure from the brake valve into mechanical force that pushes the brake shoes against the drum. It is the final actuating component in the service-brake circuit. When the seals inside the cylinder degrade, the pedal travel increases and braking force drops because fluid bypasses the piston rather than moving it. The cylinder bore is machined to match the original piston clearance, which keeps the internal leakage rate within the system's design limits. On replacement, bleed the circuit after installation to remove trapped air.