An ATV's brake master cylinder is responsible for generating the hydraulic pressure that closes the brake pads around the wheel's brake discs. The master cylinder relies heavily on the various seals that prevent the hydraulic brake fluid from leaking out of its housing. Over time, these seals can become brittle and crack, allowing the fluid to escape and reducing braking power in the process. Internal contamination can also foul the seals and damage the cylinder's pressure-generating piston. To prevent problems, a leaking or seized master cylinder can be rebuilt with new parts and basic tools.
Remove the master cylinder's fluid container cover, either by hand or with a Phillips head screwdriver to remove the cover's bolts. Pull the rubber diaphragm out of the fluid container. Suck the brake fluid out of the container with a siphon.
Unscrew the master cylinder's handlebar perch bolts with a socket wrench. Lift the master cylinder off of the handlebar. Wrap a towel around the master cylinder and remove the brake line bolt from the end of the cylinder with the socket wrench. Sop up any remaining brake fluid with a towel. Set the master cylinder on a clean work space.
Remove the brake lever's nut from the bottom of the master cylinder with the socket wrench. Unscrew the lever's pivot bolt with a flat head screwdriver. Pull the lever away from the master cylinder to access the piston.
Pull the rubber dust cover off of the master cylinder's piston. Pry out the piston's circlip with needle-nose pliers. Pull the piston and the piston spring out of the master cylinder.
Clean the master cylinder's fluid container and fluid passages with isopropyl alcohol. Blow compressed air through the master cylinder to dislodge any trapped sediment or debris.
Inspect the master cylinder housing and the piston's chamber for cracks, scratches or pitting. Replace the master cylinder if it is damaged in any way.
Coat the new piston and piston spring with clean brake fluid. Slip the piston spring into the master cylinder's piston chamber then place the piston over it. Insert a new circlip over the piston with needle-nose pliers. Push a new rubber dust boot over the end of the piston.
Reinstall the master cylinder onto the handlebar and reattach the brake lever, following the reverse method of removal. Screw the brake line bolt into place on the end of the master cylinder with the socket wrench.
Fill the master cylinder's fluid container with fresh brake fluid. Place a new rubber diaphragm over the fluid container. Screw the container's cover into place, either by hand or with the Phillips head screwdriver to tighten the cover's bolts.
