Brake fluid slowly absorbs water. It absorbs the water vapor present in air. The brake system is not sealed closed, so air is present in the master cylinder reservoir. If the system were sealed closed, as the brake pads wear and create space in the cylinders of the calipers, a vacuum would be created, which would eventually lock the system and make it not work.
Water in the fluid lowers the maximum boiling point of the fluid. When the fluid "boils" is now contains brake fluid in a gaseous state (not a liquid state - remember states of change: gas-liquid-solid - from high-school chemistry?). Once in a gaseous state, the fluid becomes compressible and does not transfer force from the master cylinder to the calipers as well as fluid in a liquid state.
And by the way to the OP, there is no "filter" in the brake system to change. Brake fluid is not like engine oil.
Last edited by Efthreeoh; 11-11-2012 at 10:03 AM.