Most motorcycle manufacturers recommend replacing brake fluid every two years. That’s because brake fluid absorbs moisture, and the more water it absorbs the lower the temperature at which it boils, which can affect braking performance under extreme conditions. Water in the fluid can also cause corrosion in the system. The routine for replacing brake fluid doesn’t vary much from bike to bike (although ABS systems might require some special dealer-only tools); here’s the general outline.
You’ll need: fresh brake fluid (buy extra, just in case); rags or towels to cover all the painted surfaces (brake fluid ruins paint); a pan or bucket for the used fluid; a box-end wrench for the bleeder screw at the caliper (not an open end––trust us on this); and a length of clear plastic tubing that fits over the bleeder nipple. Get brake fluid with the right DOT number, and don’t use silicone fluid (used in older Harleys) in a system designed for polyglycol fluids (used in pretty much everything else).
Get the front brake master cylinder reservoir as level as possible; you might have to turn the front wheel or put a block under the sidestand. Loosen––don’t remove––the bleeder screw and put one end of the tubing on it, and the other end in the pan. Pour in enough brake fluid to submerge the end of the tube so it won’t suck air back into the caliper.
For dual-disc front brakes, bleed the caliper farthest from the master cylinder first. Open the bleeder screw and pull the brake lever slowly. Before the lever bottoms out on the grip, close the bleeder. Release the lever and repeat the process, adding fresh fluid to the reservoir as the level drops, until you see clean fluid come out the tube. There will probably be some tiny air bubbles in the fluid at first. Keep pumping until they disappear. Tighten the bleeder, top up the reservoir, and either put the cap back on or move on to the second caliper. The rear brake is done the same way.
There are vacuum tools that draw the fluid out so you don’t have to pump the lever or pedal, and bleeder screws with spring-loaded check balls in them so you don’t have to loosen and tighten the bleeder bolt repeatedly. Both are handy, as is a second set of hands in case the brake lever and the caliper are awkwardly far apart.
Check your brake pads before you replace the fluid, and if you need new ones break them in right. The new pads and the rotor might feel smooth to the touch but on a microscopic level actual contact between them is less than optimal for good braking. The break-in gives them time to mate to each other by gradually wearing down the high spots on each surface so the pad contacts the rotors more fully.
The break-in drill varies by pad manufacturer. Some recommend going easy on the brakes for 150 miles or so, while others get the job done with 10 hard, tire-smoking stops as soon as the new pads go in. Before you install the pads, clean the rotors with brake cleaner to remove old pad material that transferred to the rotor. Use a Scotchbrite pad to remove stubborn spots. Dry off everything thoroughly before allowing the new pads to touch the rotors. If the rotor is scored, or has deep grooves in it, replace it. A badly worn rotor prolongs break-in, and can ruin the new pads by creating hot spots that glaze and overheat the brakes.