When brake fluid absorbs water, its boiling point is reduced. Vapour, unlike liquid, is compressible, so pressing the brake pedal merely compresses vapour instead of operating the brakes. This kind of brake failure can only be avoided by changing the brake fluid regularly. Brake fluid deteriorates not with distance but with time. It continues to absorb water even when the vehicle is stationary.
Most vehicle manufacturers recommend regular brake fluid changes on a time basis, eg.
Unfortunately, such recommendations are not always followed and independent surveys have shown that many vehicles have dangerous brake fluid in them. DOT specifications include minimum boiling point requirements and measurement of boiling point is one of the best ways to determine whether or not fluid needs changing.