by Permission of Author (Molekule) A brake fluid is first and foremost a specialized hydraulic fluid. A hydraulic fluid transmits power from one point in a vehicle (pedal/vacuum boost unit) to another (brake calipers). Hydraulic fluids used in automotive hydraulic brake systems must satisfy a variety of requirements. In manual trasmission equipped cars, this same type of fluid may transmit power from the clutch pedal’s hydraulic cylinder to the clutch actuator cylinder to activate the throw-out bearing, for clutch disengagement during gear changes. In general, the requirements for a brake fluid include chemical and thermal stability, suitable viscosities for the intended use, fluidity over the use-temperature range, low volatility, non-corrosiveness to metals, limited effect on rubber parts and good tolerance for water. Thus, a hydraulic brake fluid to be commercially acceptable is required to meet industry-accepted specifications as well as those established by governmental agencies. Industrial specifications include Society of Automotive Engineers (SAE) specifications such as 7Or1 Artic and 7Or3. Governmental specifications include National Highway Traffic Safety Administration, Department of Transportation, Federal Motor Vehicle Safety Standard 571.116 and 571.116a. Automotive hydraulic brake fluids used today are most often synthetic glycol-base, water-miscible fluids. Brake fluids generally have been blends of several components such as vegetable oils, various alcohol-based fluids, synthetic ester base oils, diluents and one or more oxidation and corrosion inhibitors. Blended hydraulic fluids have contained such lubricants as castor oil, polyoxyalkylene glycols, glycol ricinoleate, and glyceryl ethers of polyoxyalkylene glycols and such diluents as butyl alcohols, amyl alcohols, glycol esters, polyoxyalkylene glycols, monobutyl ethers, ethylene glycol monoalkyl ethers and the like. In addition to the myriad of synthetic base fluids (they can range from 7 to 11), brake fluids have anti-oxidants, corrosion inhibitors, and metal deactivators included. Not all blends have been entirely satisfactory in all cases. Those that are satisfactory with respect to most of the requirements are difficult to prepare, since a blend component that satisfies one requirement may be disadvantageous with respect to another requirement. Thus, a blend component that meets a high boiling point requirement frequently does not meet the low temperature viscosity requirement or a blend component that satisfies the low temperature viscosity requirement may adversely affect rubber parts used in hydraulic systems, e.g., cause swelling, softening, and the like. Brake fluid absorbs moisture form the atmosphere, thus it is hygroscopic. Moisture gradually reduces the boiling point, so the fluid should be changed periodically to remove water and other contaminants and to ensure the continued effectiveness of the braking system. The properties of different types of brake fluids are tested for many different characteristics such as ph value, viscosity, resistance to oxidation, and stability, and graded against compliance standards set by the United States Department of Transportation (DOT) as noted above. Brake fluid DOT specifications: DOT 2 is castor oil based; pretty much obsolete. DOT 3 is composed of various glycol esters and ethers. Boiling point: 284° F (140° C) DOT 4 is also composed of glycol esters and ethers, and boronic compounds. Boiling point: 311° F (155° C) DOT 5 is silicone-based. It is NOT recommended for any vehicle equipped with antilock brakes (ABS). It gives better protection against corrosion, and is more suitable for use in wet driving conditions. Boiling point: 356° F (180° C) DOT 5.1 is a high-boiling point fluid that is suitable for ABS-equipped vehicles. It contains polyalkylene glycol ether, but is more expensive than other brake fluids. Boiling point: 375° F (190.6° C) Even if they have similar base composition, fluids with different DOT ratings must NOT be mixed.