How to Rebuild the Front Brake Master Cylinder on a Harley Heritage

Written by kyle mcbride
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How to Rebuild the Front Brake Master Cylinder on a Harley Heritage
Rebuild your Heritage Softail's master cylinder to restore proper braking performance. (Jupiterimages/Comstock/Getty Images)

Harley-Davidson's Heritage Softail model uses a hand-powered, hydraulic brake system to control the front brakes. Squeezing the brake lever actuates a piston assembly inside of the master cylinder. The piston pressurises the brake fluid in the master cylinder, brake lines and front brake caliper, causing the caliper to compress the brake pads against the brake rotor. Age, extreme conditions and the use of improper brake fluid can degrade the rubber components in the master cylinder. Loss of performance of the master cylinder leads to an unsafe riding condition due to diminished and unreliable brake function.

Skill level:

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Things you need

  • Clear, plastic tubing
  • Catch pan
  • Wrench set
  • 5/32-inch cardboard shim
  • T27 Torx driver
  • Outside snap-ring pliers
  • Dental pick
  • Master-cylinder rebuild kit
  • Lint-free rag
  • Denatured alcohol
  • Compressed air
  • New pivot-pin snap-ring
  • Pound-inch torque wrench
  • Pound-foot torque wrench
  • D.O.T. 4 brake fluid

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  1. 1

    Place a clear, plastic tube over the front caliper bleeder valve and lead the other end of the tube into a catch pan. Loosen the bleeder valve one-quarter turn counterclockwise with a wrench. Operate the brake lever by hand until all of the fluid is purged from the system. Tighten the bleeder valve.

  2. 2

    Loosen the master cylinder's banjo bolt with a wrench. Remove the banjo bolt and two washers. Discard the washers.

  3. 3

    Squeeze the brake lever slightly. Insert a 5/32-inch cardboard insert in the gap created between the forward edge of the brake lever and the brake lever housing. Failure to use the cardboard insert will result in damage to the rubber boot and front stoplight-switch plunger.

  4. 4

    Loosen the switch-housing screws and loosen and remove the two fasteners from the master cylinder handlebar clamp with a T27 Torx driver. Pull the master cylinder and handlebar clamp from the handlebar.

  5. 5

    Remove the snap ring from the hand-lever pivot pin with a pair of outside snap-ring pliers. Push the hand-lever pivot pin up and out of the lever bracket and remove the brake lever. Discard the old snap ring.

  6. 6

    Remove the wiper (outer rubber washer) carefully with a dental pick. Slide the piston, O-ring, primary cup and spring out of the master cylinder body.

  7. 7

    Remove two reservoir cap retaining screws with a No. 2 Phillips screwdriver. Remove the reservoir cap and gasket

  8. 8

    Clean the brake parts with denatured alcohol or fresh D.O.T. 4 brake fluid. Dry them with a clean, soft, lint-free rag and compressed air.

  9. 9

    Inspect the banjo fitting mating surfaces for wear or damage. Inspect the master cylinder's piston bore for damage such as scoring or pitting. Replace the master cylinder body if either of these areas are damaged.

  10. 10

    Slide the new O-ring onto the piston until it seats in its groove.

  11. 11

    Slide the primary cup over the lip located on the end of the piston so that the smaller end of the primary cup bears on the piston shoulder.

  12. 12

    Grease the master cylinder bore, O-ring and primary cup with the special lubricant that was provided with the rebuild kit.

  13. 13

    Place the spring in its recess in the master cylinder bore. Slip the piston assembly into the bore and over the spring.

  14. 14

    Slide the wiper onto the piston cap with the flat side of the wiper bearing against the shoulder on the piston cap.

  15. 15

    Insert the piston cap and wiper into the master-cylinder bore. Push it into the bore until the wiper completely seats in its recess.

  16. 16

    Place the reservoir cover in position and secure it with the two retaining screws. Tighten the screws just enough to retain the cover. Do not tighten the screws fully.

  17. 17

    Place the cardboard spacer in its place in the brake-lever bracket. Hold the brake lever in position in the bracket and align the pivot-pin holes. Insert the pivot pin. Install the new pivot-pin snap ring with a pair of outside snap-ring pliers. Ensure that the snap ring fully seats in its groove.

  18. 18

    Place the master cylinder against the switch housing on the handlebars. Hold the handlebar clamp in position and install two clamp bolts.

  19. 19

    Tighten the handlebar clamp bolts to 60 to 80 inch-pounds with a torque wrench and a T27 Torx driver. Tighten the switch-housing screws to 35 to 45 inch-pounds with a torque wrench and a T27 Torx driver.

  20. 20

    Install a new banjo-bolt washer onto the banjo bolt, followed by the banjo fitting and the second banjo washer. Thread the banjo-bolt into the master cylinder. Tighten the banjo-bolt to 17 to 22 foot-pounds of torque with a torque wrench and socket.

  21. 21

    Bleed the brakes and top off the fluid in the master cylinder with D.O.T. 4 brake fluid.

  22. 22

    Install the reservoir cover and cover screws. Tighten the cover screws to 6 to 8 inch-pounds of torque with a torque wrench and a No. 2 Phillips screwdriver tip.

  23. 23

    Turn the ignition switch to Ignition. Operate the front brake lever and verify that the brake light is working. Test the brakes for function while sitting still before test riding the vehicle.

Tips and warnings

  • Dispose of the old brake fluid at an approved hazardous-waste facility.
  • Never mix different types of brake fluid. Use only D.O.T. 4 brake fluid. Use caution when using D.O.T. 4 brake fluid around painted parts as it can damage the finish.
  • Wear eye protection when removing or installing snap-rings or using compressed air to clean the master cylinder. Never direct a compressed air stream towards yourself as injection injuries can be serious and even life threatening.
  • Ensure that no brake fluid is allowed to contact the brake pads, rotor or tires as it can lead to diminished braking and an unsafe riding condition.

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