Brakes are critical motorcycle components.
A leaking master cylinder presents many hazards. The leak may result in diminished braking power, which can be especially critical on the rear brakes of your Harley. Brake fluid is also very corrosive and can cause serious damage to painted surfaces. In addition, brake fluid is hygroscopic (it attracts water), so a leak often introduces water into the braking system. This can cause internal corrosion of brake lines and components. Identifying and repairing a leak in the rear master cylinder is a somewhat involved but doable project for a DIY mechanic.
Finding the Leak
1. Wipe the entire area around the master cylinder clean. Have someone pump the brake pedal a couple of times while you watch carefully. If the fluid is coming from where the brake line connects to the cylinder, it may just be leaking from one of the copper washers on either side of the banjo connection. Make sure that the bolt holding the brake line on is tight, and check again.
2. Check whether the fluid is leaking around the lid of the master cylinder. This indicates a faulty or improperly seated gasket. Remove the lid and inspect the gasket. Make sure that the gasket fits evenly inside the lip of the lid and there are no gaps. Over time the gasket may become distorted, especially if the bike is stored for long periods. If the gasket is warped or misshaped, replace the gasket and test again.
3. Rebuild the master cylinder if it is leaking where the plunger from the pedal enters the master cylinder. To rebuild the master cylinder, you will need a kit from the manufacturer. At a minimum, you need to know the bore, or inside diameter, of the master cylinder to obtain replacement parts.
Rebuilding the Master Cylinder
4. Remove the brake line, using a drain pan or a cup to catch the brake fluid. If you have a painted frame, protect it with plastic carefully taped to the frame to prevent damage. Remove the master cylinder from the frame. Remove the lid and turn the master cylinder over, draining the fluid into the drain pan. Discard the old brake fluid.
5. Clear a work area and cover it with a towel or a shop rag. Push in the end of the plunger on the master cylinder with a drift, and remove the clip that holds it in place, using circlip pliers. Set the circlip aside and let the piston slide out gently. Remove the spring and the two rubber “cups” behind the piston. Inspect the bore of the master cylinder for pitted areas and dirt. Clean the entire master cylinder thoroughly using solvent or brake parts cleaner.
6. Replace the spring and the two cups with those provided in the master cylinder kit. Slide the piston back in, and holding it in with the drift, replace the circlip. Pump the piston in and out with the drift to ensure that the circlip is seated properly. Refill the master cylinder with brake fluid specified by the manufacturer. Using a bench bleeding kit, pump the piston slowly in and out using the drift, ensuring that it does not leak. Follow the bench bleeding instructions included with the master cylinder kit until you see no air bubbles in the brake fluid.
7. Reinstall the master cylinder to the frame of the motorcycle and reattach the brake hose. Bleed the rear brakes, using the manufacturer’s instructions. Make sure that bleeder bolts and brake line bolts are tight, and that there are no more leaks.
8. Test the brakes. Before taking the bike for a test ride, roll it back and forth, using only the rear brakes to stop. If the bike passes this test, take it for a test ride. Proceed slowly at first, using both front and rear brakes to stop the Harley. If the brakes feel soft and spongy, try bleeding them again, then attempt another test ride. Make sure that the brakes feel firm and stop the motorcycle securely before riding normally again.