Most hydraulic fluids have a taste for water and will gradually suck moisture past the rubber seals in your calipers. If your fluid is contaminated with water, heavy brake use will raise the temperature to the point where the water will boil (at a significantly lower level than pure fluid would), leading to brake fade and the dreaded lever-to-the-grip experience.
Cruisers should swap out hydraulic fluid at least once a season. Some people claim you should do this as part of the winterizing process, so that the moisture-free fluid sits in your bike all winter. Having never had problems develop while my bike was in storage, I prefer to flush the fluid at the beginning of the season so that I get the benefit of fresh fluid in the spring. Those whose priority is maximum braking performance will most likely want to follow this schedule, too.
The tools required for this project are minimal, but you can buy some specialized tools if you like-you know you want to. The Mityvac bleeding system (from Lockhart Phillips) is perfect for bone-dry systems-such as when you've installed stainless steel lines. If the system is already primed, an old jar and clear hose will work just fine. The fluid you add to your system should come from an unopened container. Remember the hydraulic fluid's thirst for water. Although DOT 5 fluids are silicone-based and, therefore, don't absorb moisture, some riders don't like the feel at the lever as much. Most OEs still recommend DOT 4 fluid. Also, make sure you buy a name-brand fluid. Buy more than you think you need. (If you're having trouble with bubbles in your lines, the best cure is to run plenty of fluid through the system to sweep them out.) Finally, never mix brands of brake fluids. If you need to top off the reservoir and don't know what type or brand of fluid is already in the system, flush the entire system to avoid any problems of interaction between the different formulations, even though-in theory-all DOT 4 (and DOT 5.1) fluids should play well together.
Begin with your bike on its side stand or a lift. Since brake fluid damages paint and some powdercoats, remove any vulnerable parts or cover them with rags. A preparatory step required for those using vacuum bleeders and optional (but recommended) for all others, is to wrap the threads of the calipers' bleeder valves with Teflon tape. Vacuum bleeders create so much suction that they will draw air into the system past the bleeder valves' threads, making it impossible to determine when all air has been removed from the system. Teflon tape fills the minuscule gaps between the threads when the valve is not completely closed and is a worthwhile modification to all calipers. One warning, though: Don't just remove the valve with the caliper mounted to the bike. Since the master cylinder is higher than the caliper, fluid will leak out all over the place, ruining your brake pads. Raise the caliper equal to or above the reservoir to keep the mess to a minimum. Of course, if your lines are empty-as when you've just added stainless steel ones-this is not a problem.
Rear calipers have their own idiosyncrasies to consider. Since the brake line travels horizontally from the master cylinder to the caliper, air bubbles are trapped in the line more easily. To assist bubbles in their travels, remove the caliper and hold it higher than the master cylinder, allowing the bubbles' tendency to rise to keep them moving as you run the new fluid through the system. If you're pumping the pedal, make sure you place a spacer between the brake pads to keep the piston(s) from popping out of the caliper.