Brake fluid, however, has an extremely unpleasant property: it readily attaches water, that is, it is hygroscopic. Water can get - for example, due to moisture in the air - into the brake fluid: through the reservoir, as well as through microscopic cracks in the brake hoses and rubber cuffs of the brake cylinders. Such water absorption leads not only to corrosion of the metal elements of the system, but also causes a decrease in the boiling point of the liquid. Only at 2.5% water content does the boiling point of the brake fluid reach 150°C.
This is very dangerous under heavy load on the brake system (driving with a trailer, frequent braking to a complete stop on a freeway), as the brake fluid becomes very hot. Hot brakes cause bubbles of water vapor to form in the brake fluid. These bubbles create a kind of void in the brake system, and at the same time the brake pedal travel increases; it may happen that you press your foot into «emptiness»! In this case, you need to make a few quick presses on the brake pedal. The formation of steam bubbles is especially dangerous when the machine is stopped after a heavy load on the brakes. In the absence of air cooling in a stationary state, the parts of the brake system heat up even more; they reach their highest temperature about 15 minutes after stopping. If you then continue to move, it can lead to unpredictable consequences. About half an hour after stopping, the brake fluid will be at normal temperature.
To avoid this, it is recommended to change the brake fluid every 50,000 km, but at least once every 3 years. The brake fluid must comply with SAE J 1703 specification. In Germany, this corresponds to the Super DOT 3 or DOT 4 specification.
The brake fluid level must not fall below the lower MINI mark. To check the serviceability of the control switch installed in the cover of the tank, you must press the rubber cover (shown by arrow). When you press it on the dashboard, the control lamp should light up. The parking brake must be disengaged during this test. If you suspect a malfunction, check the connector.