Brake fluid (DOT 3 / DOT 4 / DOT 5.1)
Definition
Brake fluid transmits pedal force to the calipers and must be incompressible, lubricate the ABS valves and seals, and resist boiling; DOT 3, DOT 4 and DOT 5.1 are glycol-ether based and miscible, with dry boiling points of at least 205, 230 and 260 °C respectively, while DOT 5 is silicone, incompatible with the others and never used in ABS cars. Glycol fluid is hygroscopic: it absorbs water through the hoses and reservoir, and at 3% water the wet boiling point drops to around 140-155 °C, so hard braking can boil it and produce a spongy or sinking pedal; water also corrodes ABS unit bores and calipers from the inside. Manufacturers specify replacement every two years regardless of mileage, and low-viscosity DOT 4 LV (class 6) is required on many modern ESP systems with fast-acting valves, so follow the specification on the reservoir cap. Test with a boiling point tester or a water-content pen, check for dark colour and debris, and bleed in the sequence the manufacturer specifies, often with a scan tool to cycle the ABS pump. There are no fluid-quality codes; related faults are a soft pedal, ABS valve codes C0110/C0121 from corrosion, brake warning lamp from low level (level switch), and brake pressure sensor plausibility codes C0131 when air is present.