Master cylinder & brake fluid

How it works

The master cylinder is the pump that turns the effort of your hand (or foot) into hydraulic pressure. When you pull the lever, a small piston pushes brake fluid down the hose to the caliper; when you release it, a spring returns it and the reservoir tops up the volume. Brake fluid (often DOT 4, sometimes DOT 5.1; the type is marked on the cap) has two drawbacks: it absorbs moisture from the air over time, which lowers its boiling point, and it attacks paint. Old fluid can boil under hard braking: vapour bubbles form and the lever goes spongy or even comes back to the bar. Air in the system after a repair gives the same symptom. The master cylinder's internal seals can also wear: the lever slowly sinks under steady pressure, or fluid weeps out. The seals are then replaced with a kit, or the whole master cylinder is renewed.

Maintenance tips

• ✓Check the reservoir level with the bike upright: it drops slowly as pads wear, but a fast drop means a leak.
• ✓Change brake fluid at the interval in your manual (often every 2 years), using the type on the cap, and never mix DOT 5 (silicone) with DOT 4.
• ✓Protect the paintwork with a rag before opening the reservoir: brake fluid damages it.
• ✓Spongy lever or one that reaches the bar: don't ride; have the system bled and the seals checked.
• ✓Any time the system is opened, a careful bleed is mandatory; if in doubt, leave it to a professional.