Brake caliper

The caliper is the metal body that wraps around the edge of the brake disc. It is bolted to the fork leg (front) or to a bracket on the swingarm (rear). It holds the pads and one or more pistons.

What it does

Brake fluid from the master cylinder pushes the pistons out, and they press the pads onto the disc. Rubber seals around each piston stop leaks and pull the piston back a tiny amount when you release the brake. On a sliding caliper, the body moves on greased pins so both pads press evenly. Dirt, rust and old fluid make pistons and pins stick.

Signs it is worn or failing

• Wheel drags or the caliper gets hot: Brake caliper sticks, Brake drag.
• One pad much thinner: Pads wear unevenly.
• Fluid wet around a piston: Brake fluid leak. Stop riding.
• Weak braking: Weak brakes.
• Brakes that heat up with the lever untouched, or that lock or drag, point to gum and corrosion behind a piston. Water in the fluid is the usual cause.

When to replace it

Clean and free the pistons first. Rebuild with new seals if they leak or stick after cleaning. Replace the whole caliper if the bore is pitted or the body is cracked.

What to match when buying

• Make, model, year and front or rear. Caliper brand (often Nissin or a local maker) and the number and size of pistons.
• Seal kits are sized by piston diameter; measure or use the part number on the caliper.
• Mounting hole spacing and hose thread must match.
• Bikes with CBS or ABS: the caliper may have extra ports; it must be the exact part.
• Avoid unbranded calipers with rough casting and soft bleed screws.

Fitting notes

• Clean only with brake fluid or isopropyl alcohol, never petrol or oil.
• Use new copper or aluminium sealing washers on the banjo bolt.
• Caliper bolt torque: use the value in your bike's manual for that exact bolt.
• See Clean a brake caliper and free the pistons (home) and Rebuild a brake caliper (workshop). Always bleed afterwards.
• When the caliper is off the disc, tie it up with wire or cord. Never let it hang by the hose.
• Fit the mounting bolts with thread locker and tighten them to the value in your bike's manual. On many rear calipers it is easier to take out the axle and remove the whole bracket with the caliper.

Reference (K-Light 125)