Brakes on a Harley-Davidson Fat Boy motorcycle are hand-powered, hydraulically operated systems. Pressure generated at the master cylinder is contained by the hoses and delivered to the brake calipers to effectively operate the calipers and slow the vehicle. The brake fluid may trap air bubbles and carry them into the hoses and calipers, where the bubbles cause spongy brakes and ultimately lead to unsafe riding conditions due to diminished braking capacity. Brake systems in this condition must be bled to remove the air bubbles and restore the system to full braking capacity. Fat Boy motorcycles with air in the brake system must never be ridden until the air has been bled.
- Skill level:
- Moderately Challenging
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Things you need
- Clean, soft shop rags
- #2 Phillips screwdriver
- Two feet of 1/4-inch, clear plastic tubing
- clean oil drain pan
- D.O.T. 4 brake fluid
- 5/16-inch open-end wrench
- Inch-pound torque wrench
- 5/16-inch socket
- #2 Phillips driver socket
Park the Fat Boy on firm, level ground. Have a helper mount the bike to hold it upright and level with the front wheel pointed straight ahead.
Wipe any fluid or dirt from the master cylinder cover, bleeder valve cover and surrounding area with a clean, soft shop rag.
Remove the rubber bleeder valve cover by hand.
Cover the fuel tank and front bumper with clean, soft shop rags.
Remove two screws from the top of the master cylinder cover with a #2 Phillips screwdriver.
Remove the master cylinder cover and gasket by hand. Place them in a safe, clean location.
Install one end of the clear plastic tubing onto the bleeder valve. Put the other end of the tube into the oil drain pan.
Pour D.O.T. 4 brake fluid into the master cylinder to bring the level up to within 1/4 inch of the gasket surface.
Have your helper depress and hold the brake lever gently to pressurise the system.
Open the bleeder valve 1/2 turn with a 5/16-inch open-end wrench while your helper holds the brake lever depressed. Observe the fluid flowing through the clear tubing for air bubbles. Tighten the bleeder valve once flow has ceased. Release the brake lever gradually.
Repeat steps 9 and 10 until the fluid discharging from the caliper is free of air bubbles. Observe the fluid level in the master cylinder and keep it full. Do not allow the master cylinder to run out of fluid.
Remove the clear tubing from the bleeder valve. Immediately clean up any fluid that may be present with a clean shop rag.
Tighten the bleeder valve to 80 to 100 inch-pounds with an inch-pound torque wrench and socket. Install the rubber bleeder valve cover by hand.
Operate the brake lever and observe the fluid in the master cylinder for proper bleed-back operation. A small geyser of fluid should be visible when the lever is actuated. Consult a certified Harley mechanic if bleed-back operation is faulty.
Top off the fluid level in the master cylinder to within 1/4 inch of the gasket surface.
Place the master cylinder gasket and cover in position. Install the cover screws with a #2 Phillips screwdriver.
Torque the cover screws to six to eight inch-pounds with an inch-pound torque wrench and a #2 Phillips socket driver.
Clean up spilt fluid immediately with a clean, soft shop rag. Remove all shop rags from the motorcycle.
Actuate the brake lever until resistance is felt. Ensure that the brakes are engaging and releasing before attempting to ride the bike.
Test ride the vehicle carefully at low speeds. Repeat the bleeding process if the brakes still feel spongy or weak.
Tips and warnings
- Handle D.O.T. 4 brake fluid with care. Ingestion and eye contact can cause serious medical complications. Perform this bleeding procedure in a well-ventilated area. D.O.T 4 brake fluid can also damage the paint and finish on painted parts of the Fat Boy. Protect the painted parts with clean shop rags and wipe up any spilt fluid immediately. Do not allow dirt or any foreign material to enter the master cylinder during this operation as severe brake system damage and loss of control may result.
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