How do I change the brake fluid?
Brake fluid is hygroscopic, 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. Not the color of nut brown ale:
Clear, like the flesh of a pear:
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.
Manual (no vacuum pump) Method
The process for changing brake fluid isn't difficult, but it requires a bit of time, some patience, and a few supplies. Plan on your first fluid change taking an hour and a half without a vacuum pump, or about an hour with one. You'll need the following:
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, using a vacuum pump, large syringe, etc. and then fill the reservoir with new brake fluid. 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.
Tightly seal the reservoir cap and bleeder valve to deter air entry. Torque for bleeder valve is 69 in-lbs. Repeat for the rear brake (the rear system is smaller and will probably take less time than the front). You should notice improved lever/pedal feel, and your stopping distances should decrease as well.
Alternate (vacuum pump) Method
If you can afford it, go spend US$35 on a hand-operated vacuum pump. They're available at most reasonably sized auto parts stores, and tool stores like Harbor Freight. The cheaper plastic ones work fine, although the nicer metal ones (more like US$60) are likely to last longer.
With the vacuum pump you use a different method of changing the fluid (same tools required):
First, attach the vacuum pump to the fluid reservoir which came with it. Make sure that the vacuum pump is attached to the tube which doesn't extend to the bottom of the bottle. Any time you're dealing with fluids, use the reservoir, as sucking fluids into the pump itself would be a horrible mess and could damage the pump.
Attach the pump's reservoir line to the brake caliper bleed nipple. Open the brake cylinder reservoir cover on the handlebars or by the seat. Have your bottle of new fluid handy.
A 10mm box end wrench can just hang in place if you run the tube through it. With a little care the tube won't fall off, the wrench won't fall off, and you can pay attention to the vacuum gauge and brake fluid level. There's no need to hold all the stuff while you're doing the actual work; this is to give you the big picture.
Pump up a mild vacuum: just one or two pumps. If you do it too much, you'll suck the fluid out so fast that you'll have to do it all over again, plus bleed the system. Once you have established a bit of vacuum and the bike's reservoir is open, open up the bleed nipple with the wrench. With the bleeder open, pump the brake control (lever or pedal) once or twice, just to make sure you don't miss the fluid in the master cylinder.
Look up at the bike's fluid reservoir, and if it's less than 1/3 full, close the bleeder nipple on the caliper. Top up the bike's fluid reservoir from your bottle of new brake fluid.
Pump the vacuum pump once or twice more, and open the bleeder again. Once again, you can pump the brake control a bit to help things along, but close the bleeder before the bike's reservoir gets too low. Keep filling it from the bottle of new fluid.
Repeat this process until you see the color of brake fluid going into the vacuum pump's bottle change color. It should go from dark to light. Compare the color of the fluid in the reservoir with that in the tube coming off the bleed nipple.
Once the fluid going into the vacuum pump bottle has changed colors, you're done. You can pump a little bit more through to make sure you flush out any extra old fluid. Top up the bike's reservoir, and set the cap on the reservoir. Pump the brake control to make sure it's solid. If it feels squishy or soft, you can try pumping more brake fluid through the system, but changing out the fluid like you just did should have also taken care of any air bubbles. Some softness is to be expected with the stock rubber brake lines, and can be corrected by switching to stainless steel lines.
Once you're happy with the result, and the bike's reservoir is full to the MAX line, put the cap back on and screw it down. Repeat with the other brake system. Empty the pump's reservoir into your disposable bottle, and dispose of the brake fluid according to local laws: it's toxic. Some recycling centers allow disposing of / recycling brake fluid in the same manner as used motor oil, but be sure to check first.