Brake fluid (Audi 100 C4)

This yellowish - and, by the way, poisonous and aggressive towards car varnish - liquid does not corrode rubber and metal parts. Even at minus 40°C it remains quite fluid, and its boiling point is at 260°C. But brake fluid has another very unpleasant property: it is hygroscopic. Water can actually get into the brake fluid, for example, through air humidity, through the expansion tank, as well as microscopic leaks on the brake system hoses and rubber cuffs. Complete absorption of water does not lead to corrosion of metal parts of the system, but to a strong decrease in the boiling point. With only 2.5% water content, the boiling point is already only at 150°C.

This is dangerous when the brakes are heavily loaded, because in this case they become very hot. Air bubbles may form in the working fluid near the heated brakes. Since air is compressible, the brake pedal will fall to the floor! In this case, only rapid "pumping" of the brake pedal can help. This effect is especially dangerous when braking the car after heavy load on the brakes. Due to the lack of airflow from the oncoming flow, the brake elements become even hotter; the highest temperature is reached after about 15 minutes of parking. Only after half an hour does the brake fluid temperature drop back to normal.

Therefore, the maintenance plan prescribes a preventive maintenance change of brake fluid every two years. The brake fluid must comply with the FMVSS116DOT4 standard. All fluids marked in this way can be mixed with each other without any concerns.

(A link to the original source is available on the website AUDImanual.ru)