When I worked for the chemical company in my youth we made Brake Fluid, all grades and shipped it in rail cars. From the time you started a batch with the chemical blending, thru the heating process and seperation it is ALWAYS being heavily purged with pure Nitrogen, both the top and lower ports of the reactor. If you ever lost nitrogen pressure the blend could be saved by controlled heating and vacuum stripping of the water it absorbed, but it was a slow process. Now once the product has all it's additives and is bottled the absorbortion rate slows down a little but it still is sucking in water every time it is exposed to the air. And the bike has a vent so it is pretty much always exposed to air. You should, I do, change it regularly. At least every two years and in my opinion the brake fluid as well.