Rebuilding a Master Cylinder

While you're doing the calipers consider installing a master cylinder kit.

• If the bore of the master cylinder is badly pitted, replace the master cylinder.
• If it is only lightly pitted or glazed, hone the bore with a brake hone lubricated with brake fluid. Flush thoroughly with brake fluid, or wash with hot soapy water, rinse, and dry thoroughly.
  • If you just need to clean the bore, soak it with brake fluid and use a large bore shotgun brass cleaning brush to clean the bore and remove any old brake fluid. Wal-Mart has them.
• Make sure to get any crud out of the hole in the bottom of the reservoir.
• Reassemble.
• Prime by partially filling the reservoir and pumping the lever with a finger over the banjo bolt hole until mostly bubble-free fluid comes out around your finger.
• Reconnect the lines and bleed at the calipers.
• Turn the handlebars lock to lock and tap the master cylinder with a rubber mallet to free up any last stubborn bubbles, then bleed again to remove them.
• In especially stubborn cases, you might have to take the master cylinder off the handlebar and hold it vertically and rap it.
• If all the air is out of the system and the lever still feels mushy or moves slowly under pressure, the master cylinder needs to be repaired or placed. (MCN 9/96 p. 9; Rider 10/96 pp. 70-71.)