Brake fluid

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible. Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible. Most brake fluids used today are glycol-ether based, but mineral oil (Citroën/Rolls-Royce liquide hydraulique minéral (LHM)) and silicone-based (DOT 5) fluids are also available. Brake fluids must meet certain requirements as defined by various standards set by organizations such as the Society of Automotive Engineers (SAE), or local government equivalents. The SAE has published standards J1703, J1704, and J1705, reflecting progressively higher performance for brake fluids. The International Standards Organisation has published its standard ISO 4925, defining equivalent classes 3, 4, and 5, as well as class 5-1 and class 6. Most brake fluid sold in North America is classified by the US Department of Transportation (DOT) under its own ratings such as 'DOT 3' and 'DOT 4' and these are widely used in other countries. Their classifications broadly reflect the SAE's specifications, but with local details - Alaska and the Azores for example, have different normal temperature and humidity ranges to consider. DOT 3 is equivalent to SAE J1703 and ISO class 3, DOT 4 to SAE J1704 and ISO class 4, etc. All approved fluids must be colorless or amber to be acceptable for street use in the U.S, except for DOT 5 silicone, which must be purple. While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 if the elastomers in the system accept the borate compounds that raise the boiling point, (a temperature upgrade) a vehicle that requires DOT 4 might boil the brake fluid if a DOT 3 (a temperature downgrade) is used. Additionally, these polyglycol-ether-based fluids cannot be mixed with DOT 5.0, which is silicone based. As of 2006, most cars produced in the U.S. use DOT 4 brake fluid.