Brake master cylinder

The master cylinder is the pump under the brake lever on the right handlebar, with a fluid reservoir on top. The rear brake has its own master cylinder behind the right footpeg, pushed by the brake pedal, with a small remote reservoir.

What it does

The lever pushes a piston with rubber cups through a bore filled with fluid. This sends pressure to the caliper. When you release the lever, a spring returns the piston and fluid flows back from the reservoir through a tiny port. Worn cups let fluid slip past the piston inside, so the lever sinks slowly. Dirt or swollen seals can block the return port and make the brake drag.

Signs it is worn or failing

• Lever slowly sinks when held: Brake lever slowly sinks. Stop riding.
• Lever comes back to the bar with no visible leak: Lever to bar.
• Fluid at the lever boot or reservoir cap: Brake fluid leak.
• Brake drags and frees after opening the bleed screw: Brake drag.
• Brake heats up or drags with the lever untouched: the plunger may be gummed up by water in the fluid.

When to replace it

Rebuild with a cup and piston kit if it leaks inside or out. Replace the whole unit if the bore is scratched or pitted, or the reservoir is cracked.

What to match when buying

• Make, model, year, front or rear, and the bore size (often cast on the body, for example 1/2 or 12.7 mm). A wrong bore changes lever feel and travel.
• Rebuild kits must match the bore size and brand.
• Bikes with CBS: the rear or linked master cylinder is a special unit; match the part number.
• Reservoir cap seal (diaphragm) and lever must suit the body.

Fitting notes

• Workshop job to rebuild: clean only with brake fluid or alcohol and fit cups the right way round. See Check the brake master cylinder.
• Bleed fully afterwards: Replace brake fluid and bleed the brakes.
• Keep the reservoir level when the bike is upright before opening the cap.
• Use new parts or a rebuild kit where they can be bought. Square rubber seals often clean up well and can be reused if they are not hard or damaged.
• Before opening the reservoir, set the handlebar so the master cylinder is level, and cover the tank and any paint with a wet towel.

Reference (K-Light 125)