Brake Cylinder (Master and Slave)

A brake cylinder is a cylinder with a sliding piston in a hydraulic brake system. There are two kinds. The master cylinder, operated by the hand lever or foot pedal, pushes brake fluid into the system. At the wheel, the fluid pushes the pistons in the caliper on disc brakes, or in a wheel cylinder on some hydraulic drum brakes, pressing the pads or shoes against the braking surface.

Seals around the pistons keep fluid in and dirt out. Over time pistons corrode or stick, especially on bikes that stand unused, which causes dragging brakes, uneven pad wear or a lever that slowly sinks.

Servicing means cleaning the pistons, replacing seals, or fitting a rebuild kit, then refilling and bleeding the system. A radial master cylinder is a common upgrade for better lever feel.