How do I change the brake fluid?
Stuff you need to know
You may get some more tips from the brake bleeding article.
If your rubber brake lines are four years old or more, it's time to change them to stainless steel lines anyway, so you might as well do that. It'll save you having to replace the fluid twice.
Brake fluid is, which means that it tends to absorb any water it encounters (usually from the air). Water which contacts brake system components will tend to corrode it. Watery/old brake fluid also boils at a lower temperature, which is very dangerous in emergency stops, where a great deal of heat can be generated in the brake system. Boiling brake fluid creates pockets of vapor, which drastically reduces your ability to transmit pressure to the brakes.
For these reasons, brake fluid needs to be changed about every 2 years. As brake fluid absorbs water, it gets darker, so if your brake fluid looks dark (tan or brown), it needs to be changed. New DOT4 brake fluid is nearly clear, with a slight amber hue, although different brands and even batches will have differences in color.
Or you can follow Wes's E-Z guide to brake fluid color:
Brake fluid is corrosive to painted surfaces and will damage the paint in short order. If you spill any on your bike, wipe it up quickly. DO NOT spill any brake fluid on the brake disc or pads. If you do, use brake cleaner to clean the disc, and replace the pads. Any brake fluid (or any other fluids) on the ground can be cleaned up by sprinkling kitty litter on it.
Tools & supplies
What kind of brake fluid?
DOT 4 fluid has a slightly higher boiling point than the older DOT 3 specification. Find a quality brand of DOT 4, such as Valvoline Synthetic DOT 3&4.
Don't use DOT 5. It is completely not compatible with your brake system and could cause MASSIVE FAILURE. You can't even drain the DOT 4 and then put in DOT 5, as the seals/lines will fail. There's no reason to, anyway. DOT disc brake bikes don't usually need high temp brake fluid (like DOT 5) for a couple of reasons.
If you're racing a bike, go ahead and get the highest temp fluid you can afford. But for street bikes, just use the Valvoline Synthetic.
All you need to change your brake fluid are a screwdriver for opening the reservoir, an open-end wrench for the bleed nut, a length of hose, and some catch container (glass, such as a Snapple bottle). Once the reservoir is cleaned of the old fluid and refilled, just squeeze the lever, open the bleed nut until the brake lever hits the grip, tighten the bleed nut and release the lever. Repeat as necessary.
Put the bike on the centerstand, attach the hose to the front bleeder valve, and run the length of hose into the empty bottle.
Carefully clean the reservoir cap to prevent getting any dirt in the system. Then, unscrew and remove the reservoir cap on the right handlebar, and remove the secondary cap and rubber seal; remove as much old fluid as you can by sopping it out with a paper towel. Then, fill the reservoir with new brake fluid and place the cap back on, to avoid splashing. To swap the brake fluid, you must repeat the following sequence several times:
Keep doing this until the fluid coming into the catch bottle is the color of the new fluid. Once the color changes, you've successfully swapped out the brake fluid. When you're in the midst of bleeding, don't overtorque the bleeder valve as you close it - just close it hard enough to stop fluid from flowing.
Keep going until the brake control feels firm and solid. The brake control should not sink at all and should come to a stopping point and stay there, no matter how long you hold it. If you can't get to this point, follow the instructions on bleeding the brakes.
Tightly seal the reservoir cap and bleeder valve to deter air entry. Torque for the bleeder valve is 69 in-lbs. Take it for a slow test ride, and if everything is good repeat for the rear brake. Don't do front and rear without testing in between. You should notice improved lever/pedal feel, and your stopping distances should decrease as well.
The rear system is smaller and will probably take less time than the front. Note that the rear has its own fluid reservoir, down near the brake lever. Remove the right side panel for access.