Rebuild A Harley Master Cylinder

Harley-Davidson master cylinders (m/c) are a crucial component to the safe operation of the motorcycle. The m/c houses the piston assembly and the brake fluid reservoir. When actuated by the hand or foot lever, the piston pressurizes the hydraulic system which then acts on the caliper piston, causing it to clamp the brake pads onto the brake rotor. This action converts the rotation force into heat energy, which is dissipated by the rotor. The rubber seals on the piston are prone to failure over time and are considered a critical wear point that must be repaired occasionally.

Instructions

1. Remove the hydraulic line from the m/c and discard the crush washers. Remove the m/c from the bike. The rear m/c may be simply unbolted from the frame. The front m/c must have the pivot removed from the assembly. Remove the snap-ring from the pivot and draw the pivot from its boss. Remove the hand lever from the m/c. Remove the reservoir cap and dump the brake fluid. Inspect the reservoir for silt or debris and wash with clean DOT 5 fluid if necessary.

2. Clamp the m/c in a bench vise with “soft jaws,” i.e., either padding the vise with an aluminum angle or shop rags applied to the vise jaws. This will prevent the m/c finish from being damaged by the steel vise jaws. Use caution when applying the clamp pressure as a bench vise can crack and crush the aluminum m/c if over-tightened.

3. Position the screwdriver in the piston detent. Apply pressure directly in line with the piston travel direction to compress the return spring and “unload” the snap-ring. Remove the snap-ring from the m/c and discard it. Ease off the screwdriver pressure and allow the return spring to push the piston out of the cylinder. Pull the piston free of the cylinder. Inspect the cylinder bore for pitting or scoring. Damaged cylinder bores must be replaced. Never try to hone a cylinder bore to remove damaged surfaces as it will increase the inside diameter of the bore and could lead to catastrophic brake failure.

4. Inspect the bleed-back hole inside the reservoir for obstructions or debris, and clean it if necessary. Replace the rubber components on the piston with the new pieces. Lightly lubricate the new rubber with clean brake fluid. Insert the rebuilt piston back into the m/c bore. Push it in and compress the return spring with the screwdriver. Install the new snap-ring and insure that it snaps fully into its groove. Release the piston and wipe the assembly with a clean shop rag.

5. Reinstall the brake lever and the pivot, then install a new snap-ring into the pivot retainer groove (if applicable). Reinstall the m/c onto the bike. Install the hydraulic line onto the m/c with new crush washers. There should be one washer on each side of the hydraulic line fitting with the fitting sandwiched in between. Torque the banjo bolt to factory specifications. Do not over-torque it as the banjo bolt is hollow and easily broken.

6. Fill the brake reservoir to the “Full cold” mark. Bleed the brakes to remove any air bubbles present in the system. With the reservoir cap off, observe the bleed-back hole while pumping the brakes. There should be a small geyser of fluid shooting up from the hole. The absence of a geyser indicates an improperly assembled piston or a bleed-back obstruction. This condition will cause the brakes to engage and prevent them from releasing, leading to brake overheating and/or lockup.

7. Top off the reservoir to the “Full cold” mark and replace the reservoir cap. Push the bike and actuate the brakes to insure proper operation and feel before test riding under power.