Brake fluid is hydroscopic, meaning it absorbs water. As it does this, it changes color from clear to very dark. This is an indicator as to how much water the fluid has absorbed and a great 'yardstick' by which to gauge when to change the fluid; anything darker than, say, tan and the fluid should be changed.
That applies to brake systems but not necessarily the clutch because clutch fluid always gets darker, faster than brake fluid. The reason is that the hydraulic parts internal to the clutch move much more and so wear much more than brake parts. Some of the darkness of clutch fluid is actually metal fines worn off the master and slave cylinders. Brake systems are effectively zero movement systems in which the master cylinder is used only to pressurize the system, not really move anything. Of course the brake pads do move but it is only a few thousandths of an inch. The clutch on the other hand is always used for its full travel whenever it is used; both the master cylinder and slave cylinder move throughout their entire travel each time the clutch is used. Add to that that the clutch is used more often than the brakes and it ends up that there is a lot more piston travel, and resulting wear, in the clutch than either or both brake systems.
This was my clutch slave cylinder after something like 50K miles- the areas showing wear have had some steel removed in the form of fines (extremely small particles) and it ends up in the clutch fluid because it cannot go anywhere else.
Brian
I believe the manual says change the clutch and brake fluid every two years. I noticed on my morning ride, that the color of the brake fluid still looks clear while looking at it through the little site window. but the clutch fluid is very dark, almost black looking through the plastic window.
only had the bike for 10 month, almost 10k miles on it.
should the clutch fluid be changed more often or just go the two years?