Master cylinder

The master cylinder sits at the brake lever, rear brake pedal or hydraulic clutch lever. When you pull the lever, a piston inside pushes fluid through the hose to the brake caliper pistons or to the clutch slave cylinder. A reservoir keeps the system topped up as pads wear.

Piston diameter decides the feel: a smaller piston gives more power with more lever travel, a larger one gives a firmer, more direct lever. Radial master cylinders, common on sports bikes, push the piston in line with the lever.

A spongy lever usually means air in the system or old fluid. A lever that slowly sinks to the bar points to worn piston seals. Change DOT fluid every two years or as the manual says, and never mix DOT 5 silicone fluid with glycol-based fluids.