Corrosion and contamination of the intricate and expensive ABS and DSC hardware is to be absolutely avoided. Another factor is the elevated operating temperatures encountered in the high-performance braking systems BMW’s and other manufactures use, which makes maintaining a high boiling point critical to safety. A third reason is that brake fluid is hydroscopic. This means that it can absorb water, even directly out of the air. When water gets into your brake system, the brake fluid can not function properly, leading to poor braking or even complete brake failure.
When air (or vapor) becomes present within the lines, it creates inefficiencies within the system because, unlike liquid, air can be compressed. So when enough air fills the lines, input at the pedal merely causes the air to compress instead of creating pressure at the brake corners. In other words, when air is present within the system, the efficiency and effectiveness of the braking system is reduced. Usually, a small amount of air within the brake system will contribute to a “mushy” or “soft” pedal (since less energy is required to compress the air than is required to move fluid throughout the brake lines.) If enough air enters the brake system, it can result in complete brake failure.