How often do you think the brake fluid on a professional race bike gets changed and bled? If you answered every race, you would be correct. How often do you change and bleed the brake fluid on your bike? If you haven’t serviced your brakes in the last year, you are overdue. Brake fluid goes bad because it attracts water, gets contaminated or overheats. Fresh brake fluid is light in color and, like gasoline, it becomes darker when it begins to go bad. Off-road bikes usually use DOT 3, 4, or 5.1 grade brake fluid. In general the higher the number the better the quality and the higher the boiling point. Brake fluid will attract water after a bottle has been opened so it is best to use a new sealed bottle whenever you replace your brake fluid.
Start by looking over the general condition of your entire brake system. Replace worn brake pads before you bleed your brakes. If you bleed your brakes with worn out pads you will add more oil to your system than normal and can cause your brakes to lock if you install new pads later. Replace your brake pad springs and retainers if they are worn or damaged. If your brake pin is worn replace it or sand off the corrosion to restore the polished finish. Rotors need to be straight and checked for uniform thickness. If your brake levers are bent or pivot bushings are worn replace them to restore the feel and action.
The easiest and most effective way to bleed your brakes is to reverse bleed your system. Since air always rises, it is more effective to push the old fluid and trapped air from your calipers at the bottom to your master cylinder at the top. Fasten a short length of hose to a syringe and fill it with fresh brake fluid. Brake fluid can damage plastic and paint so wrap the brake reservoir and bleeder with a towel. Brake fluid is also toxic so wear gloves when you work with it. Hold the syringe upside down so any trapped air will flow upwards toward the plunger and away from the hose. Open the brake bleeder and push the fresh fluid into your caliper until you see it come out in the master cylinder.
Drain off the excess fluid in the master cylinder by siphoning it out with a second syringe. Continue pushing fresh fluid into the cylinder until it displaces all the old fluid. Close the bleeder and set the fluid height in the reservoir about 3/4 full or to the top of the sight glass window. Pump the brake lever until it has it has a firm, solid feel.
If your brakes still feel spongy after bleeding you should check the key areas for problems. Air frequently gets trapped in and around the banjo bolts, so simply loosen the bolts to release air pockets and re-bleed as necessary. Make sure the banjo bolt washers are not leaking. Check for any leaks along your brake lines. Often the seals on your master cylinder piston will get worn out or damaged and need to be replaced. The same is true for the seals around the pistons in your caliper. Sometimes, simply pumping the lever a good number of times will restore the positive feel.
If you want your lever to have a really tight feel you can bleed off any remaining microscopic air molecules in the system by securing the brake lever to the handlebar with a zip tie overnight. Clean off any excess fluid you may have gotten on the caliper and master cylinder with contact cleaner.
Finally, clean your rotors with contact cleaner and a lint free cloth to complete the job.
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