How does air and moisture intake reduce the braking performance?
Air is a gas, gases are compressible whereas fluids are pretty much incompressible. If there is air within the brake system some of the pressure you exert at the lever will be wasted as the air is compressed; under such circumstances your brake lever can feel 'spongy' and non consistent. DOT brake fluid is hygroscopic, which means it absorbs water from the atmosphere at normal humidity levels over a period of time. This water contaminated brake fluid will have a lower overall boiling point than fresh DOT brake fluid. As we know brake fluid can become very hot under heavy use and can eventually 'boil'.