If your Shimano bicycle brakes are squealing more often and you have noticed a downturn in performance, you should replace them immediately. Brake pads on traditional Shimano brake systems like caliper (road bike), or cantilever (mountain bike) brakes wear down from rubbing against the wheel walls. In Shimano disc brake systems, the pads wear down from rubbing against the rotor. Once the groves on the pads have worn away, it is time to change them. The pads in a disc brake system cannot be seen until the brake shoe is removed, but if you have noticed a lack of performance, they probably need changing.
Turn the quick-release lever on the caliper brake to the open position. If you are changing a cantilever brake, squeeze both sides of the brake and lift out the cable guide from the brake body.
Remove the fixing screw from the caliper brake shoe using a Phillips screwdriver. Use needle-nose pliers to remove the fixing pin from the cantilever brake shoe.
Change the pads in the same manner for both brake setups. Push the brake pad out of the brake shoe from the back. Repeat this step for the second pad.
Clean the brake shoes using lubricant. Spray the lubricant over the brake shoes and wipe them off with a cloth.
Slide the new brake pads into the brake shoes. An "L" will mark the left pad, and an "R" will mark the right pad.
Replace and tighten the fixing screw (caliper), or push the fixing pin (cantilever) back into the side of the brake shoe. Repeat this step for the second brake shoe.
Return the quick-release lever on the caliper brake to the closed position, or squeeze the sides of the cantilever brake, and reattach the inner cable guide to the brake body.
Test your new brake pads before riding by spinning the wheel and pressing the corresponding brake lever. If the wheel does not stop, tighten the brake cable.
Remove the appropriate wheel using the quick release mechanism.
Straighten the split pin using needle-nose pliers, and remove it from the caliper. Unscrew the retaining pin (sometimes called the adjusting screw) using a 3-mm Allen key.
Push the brake shoe out of the caliper using a slotted screwdriver.
Remove the old brake pads and the return spring from the brake shoe. Clean the caliper, brake shoe and the return spring using a spray lubricant and a cloth.
Place the new brake pads and return spring into the brake shoe, and slide it back into the caliper. Secure the brake shoe by screwing in the retaining pin and replacing the split pin, which you must bend back into place.
Put the wheel back on. Make sure the rotor slides into the caliper and does not touch the brake pads. Tighten the retaining pin to ensure that the brake pads clear the rotor by 0.2 to 0.4mm.
Test your brakes by spinning the wheel and pulling the corresponding brake lever. If the wheel stops spinning, your brakes work and it is safe to go for a ride. If the wheel is too slow in stopping, tighten the retaining pin and test your brakes again.