Jerry,
Why would the "official" maintenance schedule be set for every two years to change brake/clutch fluids? Is this just another way of HD drumming up business? I don't know of anyone ever doing this to their automobiles on a regular basis...what would make a bike command such a schedule like that? Am I missing something here? My pickup has a hydraulic clutch set up and the thought never occurred to me to ever change fluids in the 90k miles that I have driven it. (insert head scratching noises here)
ez
ez,
Basically, if the system remains totally leak free and isn't opened (check fluid levels with the sight glass only), a two year interval is overly anal. However, since the non-ABS bikes have very simple hydraulic systems and use just a small volume of fluid, it's no big deal to flush and replace the fluid. People or shops with pressure bleeders can basically just pump new fluid in one end and collect the old fluid at the other end at the same time. If done right, you won't even introduce any air into the system.
The reason for the overly cautious schedule has to do with the fact that DOT3 and DOT4 brake fluid attracts and holds moisture, and the manufacturer wants to CYA in case of any liability claims. Any time the fluid is exposed to the atmosphere, it will "suck" moisture right out of the air. That moisture causes two major problems, the first being to lower the boiling point of the brake fluid. If the brake fluid boils, you don't get full pressure applied to the brake pads and your stopping distances get much longer. The second problem is that the moisture in the fluid tends to cause corrosion in the components of the system (ever had a brake caliper where the piston became stuck?). So even though most folks don't flush and refill the brake systems of their vehicles, it is a good idea to do so. And if you do a lot of mountain driving or towing it is even more critical. As for the hydraulic clutch, obviously you don't have the safety issue like with the brakes, but you still have the corrosion potential.
Those with DOT5 (silicone) fluids also need to look at flushing and refilling occasionally. While DOT5 doesn't attract moisture like DOT3/4, if any moisture does get into the system if will separate out at certain points in the calipers, cylinders, and lines and cause corrosion issues.
Jerry