Photos by Adam Booth
How often do you think the brake fluid on a professional racebike gets bled and changed? If you answered, “Every race,” you are absolutely correct. How often do you change and bleed the brake fluid on your bike? If you haven’t serviced your brakes in the past year, you are very overdue and should change it—even if you haven’t been riding it. Racers should bleed the brakes every 20 hours and recreational riders every 40 hours of riding.
Brake fluid goes bad because it absorbs 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 either DOT 3, 4, or 5.1-grade brake fluid. Use what your bike is spec’d for, but in general, the higher the number, the better the quality and the higher the boiling point (yes, brake fluid can boil under extreme conditions). Brake fluid will absorb water molecules in the air after a bottle has been opened, so it is best to use a new sealed bottle whenever you replace your brake fluid. At the longest, only save an opened bottle for six months.
Start by inspecting the general condition of your entire brake system. Replace worn brake pads (in most cases 1mm left of pad material means it’s time to replace) before you bleed your brakes, and replace your brake pad springs and retainers if they are worn or damaged. If the brake pin is worn or scored, replace it. If it has corrosion, replace it or sand the corrosion off to restore the polished finish. Brake rotors need to be straight and checked for thickness. You can spin the wheel and eyeball it for major warping or remove the rotor and place in on a flat surface (glass works well). Measure with a micrometer for thickness; your owner’s or shop manual will give you the thickness range your rotor should be in. If your brake levers are bent or their pivot bushings are worn, replace them to restore their proper feel and action. Wear gloves when you work with brake fluid; it is nasty stuff that is very corrosive. Wrap the brake reservoir and bleeder with a towel so any excess fluid that might spill doesn’t damage plastic and paint.
Make a brake fluid catch bottle out of an empty plastic water bottle, a short length of hose, and some heavy wire to hang the bottle. Fasten the hose to the end of the bleeder on the caliper and hang the bottle above the bleeder (if working alone) so air can rise out of the system. Remove the cap off your master cylinder, and top it off with fresh fluid. Slowly pump the brake lever a few times, and then as you push down on the lever, open the bleeder one-quarter turn, being careful not to allow any air bubbles back into the caliper. Hold the brake lever fully engaged and close the bleeder. Pump the brake lever a number of times with the bleeder closed, and refill the master cylinder with brake fluid. Repeat until the brake fluid coming out of the bleeder is clean and free of air bubbles. You can “snap” out the brake lever and tap the line during this to jar out any air bubbles that might be left. Clean off any excess fluid you might have 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.
Another good method to bleed your brakes is to reverse bleed your system. Since air rises, it is very 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. Hold the syringe upside down so any trapped air will flow upward 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 and you have no air bubbles. Close the bleeder and set the fluid height in the reservoir about three-quarters full or to the top of the sight glass window. Pump the brake lever until it has a firm, solid feel.
If your brakes still feel spongy after bleeding, you should check for problems in a couple of areas; one is the banjo bolts. 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 in new condition and not leaking. Check for any leaks along your brake lines. Often the rubber cups on the master cylinder piston will wear out or become damaged and need to be replaced. The same is true for the seals around the pistons in your caliper. If your brakes are not retracting, your brake fluid level might be too high or your caliper piston seals are worn, not allowing the pads to self adjust. You can give your lever a tighter feel by securing the brake lever to the handlebar with a zip-tie overnight to bleed off microscopic air molecules in the system. Because of multiple leading edges, wave-type rotors and brake pads with slots increase braking performance. They also provide improved heat and water dissipation. Brake pads often use shims between the pads and the calipers to dissipate heat and prevent brake squeal. Pads should be replaced when they become overheated; the extreme heat changes the composition of the pad material, so merely sanding off the glaze will not provide improvement to braking performance.