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 to pressure and increase braking force . This works because the liquids are not compressed enough. Most brake fluids used today are glycol-ether based, but mineral oil ( Citron / Rolls-Royce Liquid Hydraulic Mineral ( LHM )) and silicone – based (DOT 5) fluids are also available. 
Brake fluids must meet certain requirements defined by various standards set by international, national or local organizations or government agencies.
The International Standards Organization has published its standard ISO 4925, along with Class 5.1 and Class 6, defining classes 3, 4, and 5,  reflecting progressively higher performance for brake fluid.
The Society of Automotive Engineers has published SAE standards J1703, J1704, and J1705, reflecting progressively higher performance for brake fluid. They have equivalents in the international standard, ISO 4925
United States of america
The Federal Motor Vehicle Safety Standard (FMVSS) under FMVSS standard number 116  defines DOT 3, DOT 4, DOT 5 and DOT 5.1 grades, where DOT refers to the US Department of Transportation . These are widely used in other countries. Their classifications largely reflect the specifications of the SAE, but with local descriptions – Alaska and the Azores , for example, have separate 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. 
Except for DOT 5 silicone, which must be purple, all approved liquids must be colorless or amber to be acceptable for use outdoors in the US. 
While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 (temperature elevation) if the elastomers in the system accept borate compounds that raise the boiling point, [ citation needed ] a vehicle that requires DOT 4. If required, brake fluid may boil if a DOT 3 (Temperature Downgrade) is used. Additionally, these polyglycol-ether- based liquids cannot be mixed with DOT 5.0, which is silicone based.
As of 2006 , most cars produced in the US use DOT 4 brake fluid.
DOT 5 is a silicone based fluid and is different from the series of DOT 2, 3, 4, 5.1. It is immiscible with water, and with other brake fluids, and should not be mixed with them. The fluid can only be changed after a complete system change, such as a total restoration.
It contains at least 70% by weight of Diorgano Polysiloxane.  Unlike polyethylene glycol based liquids, DOT 5 is hydrophobic .  An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wide temperature range. Another quality is that it does not damage the paint. [ citation needed ]
DOT 5 brake fluid is not compatible with the anti-lock braking system . DOT 5 can circulate fluid when the anti-lock brake system is activated. DOT 5 brake fluid absorbs a small amount of air which requires care when bleeding the air system.
Lack of acceptance of silicone-based fluids led to the development of DOT 5.1, a liquid that gives the performance advantages of silicones, while retaining some familiarity and compatibility with glycol ether fluids. DOT 5.1 is the non-silicon version of DOT 5, which is defined by FMVSS 116 as being less than 70% silicon. Above that threshold makes it DOT 5.
Citroen Hydropneumatic Suspension
In the 1950s, Citron introduced a hydropneumatic suspension system, driven by an engine-driven pump and also used to operate the braking system. It used Citroen-specific hydraulic fluid. The first fluids were of variable chemistry, and were available from a variety of suppliers. Shell Donax D, Lockheed HD19, Castrol HF were some of them. Citron then attempted to improve and standardize the fluid in 1962 with the LHS ( Liquide HYDRAULIQUE Synthétique ) , a vegetable/synthetic based fluid. In 1964 it was improved with the fully synthetic LHS2. In 1966, Citroen introduced the LHM ( Liquid Hydraulic Mineral), which is a mineral fluid. The LHS was hygroscopic and gave problems with internal corrosion. Although the two fluids are incompatible, the LHM has been universal since 1967, and some older cars have been converted to use it. 
This system was also used on Rolls-Royce and some Maserati models.
Hydragas and hydroelastic suspension
Hydragas and hydroelastic suspension was a widely used form of hydropneumatic suspension, designed by Alex Moulton , and used on British Leyland cars from the 1960s . This system was not engine driven and did not include a braking system.
The fluid was a low viscosity fluid based on diluted alcohol.
49% alcohol49% Distilled Water1% Triethanolamine Phosphate (Surfactant)1% sodium mercaptobenzothiazole (stenching agent)
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to function properly.
For reliable, consistent brake system operation, brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme cold. This is especially important in systems with anti-lock braking systems (ABS), traction control and stability control (ESP), as these systems often use micro-valves and require very rapid activation.  DOT 5.1 fluids are specified with low viscosity over a wide range of temperatures, although not all cars fitted with ABS or ESP specify DOT 5.1 brake fluid.  For faster response of ABS and ESP systems, DOT 4 and DOT 5.1 brake fluids are present with lower viscosity than ISO 4925.Meet the maximum 750 mm2 /s viscosity at 40 °C °F requirement of Class 6 .  These are often named DOT 4+ or Super Dot 4 and DOT 5.1 ESP.
Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disc brake calipers. It must have a high boiling point to avoid evaporation in the lines. This vaporization creates a problem because the vapor is highly compressible relative to the liquid, and therefore negates the hydraulic transfer of braking force – so the brakes will fail to stop the vehicle. 
Quality standards refer to the “dry” and “wet” boiling points of a brake fluid. The wet boiling point, which is usually very low (though above most normal service temperatures), refers to the boiling point of a liquid after absorbing a certain amount of moisture. This is a multiple (single digit) percentage, which varies from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water-absorbing), meaning they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic liquids (such as silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the service life of the fluid.
Silicone based fluid is more compressible than glycol based fluid, giving brakes with spongy feel.  This can potentially lead to phase separation/water pooling and freezing/boiling in the system over time – mainly because of the use of single phase hygroscopic fluids. [ citation needed ]
|dry boiling point||wet boiling point [A]||Viscosity at -40°C°F||Viscosity at 100 °C (212 °F)||Primary components|
|dot 2||190 °C (374 °F)||140 °C (284 °F)||?||?||castor oil/alcohol|
|dot 3||205 °C (401 °F)||140 °C (284 °F)||1500 mm 2 / s /||1.5 mm 2 /s||glycol ether|
|dot 4||230 °C (446 °F)||155 °C (311 °F)||1800 mm 2 /s||1.5 mm 2 /s||glycol ether/borate ester|
|dot 4+||230 °C (446 °F)||155 °C (311 °F)||750 mm2 / s||1.5 mm 2 /s||glycol ether/borate ester|
|LHM+||249 °C (480 °F)||249 °C (480 °F)||1200 mm 2 /s ||6.5 mm 2 / s /||mineral oil|
|dot 5||260 °C (500 °F)||180 °C (356 °F)||900 mm 2 /s||1.5 mm 2 /s||Silicone|
|dot 5.1||260 °C (500 °F)||180 °C (356 °F)||900 mm 2 /s||1.5 mm 2 /s||glycol ether/borate ester|
|dot 5.1 esp||260 °C (500 °F)||180 °C (356 °F)||750 mm2 / s||1.5 mm 2 /s||glycol ether/borate ester|
- ^ “Wet” is defined as 3.7% water by volume
Brake fluids must not corrode metals used inside components such as calipers, wheel cylinders, master cylinders and ABS control valves. They should also be protected from corrosion as moisture enters the system. To accomplish this, additives (corrosion inhibitors) are added to the base fluid. Unlike glycol-ether based DOT fluids, silicone is less corrosive to paintwork. 
The advantage of Citron LHM mineral oil based brake fluid is the absence of rust. Seals can wear out at high mileage but otherwise these systems have exceptional longevity. It cannot be used as a substitute without replacing the seal due to incompatibility with rubber. 
Brake fluid must maintain a low level of compression even with varying temperatures to accommodate different environmental conditions. This is important to ensure consistent brake pedal feel. As compressibility increases, more brake pedal travel is required for the same amount of brake caliper piston force.
Service and maintenance
Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water-absorbing), meaning they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (such as silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the service life of the fluid. Ideally, silicone fluid should only be used to fill non-ABS systems that have not previously been filled with glycol based fluids. Any system that has used a glycol-based fluid (DOT 3/4/5.1) will contain moisture; Glycol fluid disperses moisture throughout the system and contains corrosion inhibitors. silicone fluid does not allow moisture to enter the system, But doesn’t stretch anything that already exists. Systems filled with dry silicone fluid do not require fluid replacement at intervals, only if the system has been disturbed for repair or refurbishment of a component. The United States Armed Forces has standardized on silicone brake fluid since the 1990s. Silicone fluid is widely used in cold climates, especially in Russia and Finland.
Brake fluids with different DOT ratings may not always be mixed. DOT 5 should not be mixed with any other because mixing glycol with silicone fluid can cause corrosion due to trapped moisture. Do not mix DOT 2 with anyone else. DOT 3, DOT 4, and DOT 5.1 are all based on glycol esters and can be mixed, although it is preferable to completely replace existing liquids with new ones to achieve the specified performance.
Brake fluid is toxic  and can damage painted surfaces. 
Castor Oil Based (Pre-DOT, DOT2)
- Castor oil
- alcohol, usually butanol (reddish/crimson fluid) or ethanol (yellow fluid) (methanol)
Glycol-Based (dot 3, 4, 5.1)
- alkyl ester
- aliphatic amine
- diethylene glycol
- Diethylene Glycol Monoethyl Ether
- Diethylene Glycol Monomethyl Ether
- Dimethyl Dipropylene Glycol
- polyethylene glycol monobutyl ether
- Polyethylene Glycol Monomethyl Ether
- polyethylene oxide
- Triethylene Glycol Monobutyl Ether
- Triethylene Glycol Monoethyl Ether
- Triethylene Glycol Monomethyl Ether
Silicone based (dot 5)
- Di-2-Ethylhexyl Sebacate
- dimethyl polysiloxane
- tributyl phosphate