Brake fluid
Encyclopedia
Brake fluid is a type of hydraulic fluid
used in hydraulic brake
and hydraulic clutch
applications in automobile
s, motorcycle
s, light truck
s, and some bicycle
s. It is used to transfer force into pressure. It works because liquid
s are not appreciably compressible - in their natural state the component molecules do not have internal voids and the molecules pack together well, so bulk forces are directly transferred to trying to compress the fluid's chemical bonds.
Because oils damage rubber seals and hoses in the braking system, brake fluids are not petroleum-based. Most brake fluids used today are glycol-ether
based, but mineral oil (Citroën liquide hydraulique minéral LHM
) and silicone (DOT 5
) based fluids are also available. Brake fluids must meet certain requirements as defined by various standards set by organizations such as the SAE, or local government equivalents. For example, most brake fluid sold in North America is classified by the US Department of Transportation
(DOT) under their own ratings such as "DOT 3
" and "DOT 4
". Their classifications broadly reflect the concerns addressed by the SAE's specifications, but with local details - Alaska
and the Azores
have different normal temperature and humidity ranges to consider, for example. Many countries defer explicitly to the SAE specifications, or simply refer to "best practice" which in practice would defer to the SAE.
Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent, varying from formulation to formulation. Glycol-ether/dot three/dot four/dot five point one brake fluids are hygroscopic
(water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone
/DOT 5
-based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life, although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.
Wet boiling point defined as 3.7% water by volume.
(ABS), traction control, and stability control (ESP).
diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high. Electronic testers and test strips are commercially available to measure moisture content. The corrosion inhibitors also degrade over time. New fluid should always be stored in a sealed container to avoid moisture intrusion.
DOT 5 is silicone fluid and the above does not apply. Ideally, silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol based fluid will contain moisture; glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climate, particularly in Russia and Finland.
A small drop in brake fluid level in the master cylinder reservoir can be "topped up" but if the level consistently drops, the cause should be investigated and repaired. Brake fluid level in the master cylinder will drop as the linings (pads or shoes) wear and the calipers or wheel cylinders extend further to compensate. Overspill from pushing back pistons should be avoided, because glycol based fluid will quickly lift or strip paints and other coatings on contact (it can be removed by quickly washing with water, not wiping). Brake fluid level may also be low because of a leak, which could result in a loss of hydraulic pressure and consequently a significant loss of braking ability. Modern cars have split hydraulic circuits to ensure against total hydraulic failure. Brake fluids with different DOT ratings should not be mixed; not all DOT fluid is compatible. This is because it will dilute and reduce the properties of the higher specification DOT fluid, or in the case of mixing of glycol with silicone fluid may cause corrosion due to trapped moisture.
Brake fluid can be dangerous as it is toxic and flammable but has a high flash point. It can become explosive in the presence of Chlorine powder and acts as a mild paint remover if left on painted surfaces.
Hydraulic fluid
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
used in hydraulic brake
Hydraulic brake
The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the...
and hydraulic clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
applications in automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s, motorcycle
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
s, light truck
Light truck
Light truck or light duty truck is a U.S. classification for trucks or truck-based vehicles with a payload capacity of less than 4,000 pounds...
s, and some bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
s. It is used to transfer force into pressure. It works because liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s are not appreciably compressible - in their natural state the component molecules do not have internal voids and the molecules pack together well, so bulk forces are directly transferred to trying to compress the fluid's chemical bonds.
Because oils damage rubber seals and hoses in the braking system, brake fluids are not petroleum-based. Most brake fluids used today are glycol-ether
Glycol Ethers
Glycol ethers are a group of solvents based on alkyl ethers of ethylene glycol commonly used in paints. These solvents typically have higher boiling point, together with the favorable solvent properties of lower molecular weight of ethers and alcohols. The word "Cellosolve" was registered in 1924...
based, but mineral oil (Citroën liquide hydraulique minéral LHM
Citroën DS
The Citroën DS is an executive car produced by the French manufacturer Citroën between 1955 and 1975. Styled by Italian sculptor and industrial designer Flaminio Bertoni and the French aeronautical engineer André Lefèbvre, the DS was known for its aerodynamic futuristic body design and innovative...
) and silicone (DOT 5
DOT 5
DOT 5 is one of several designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.DOT 5 is a silicone-based brake fluid ....
) based fluids are also available. Brake fluids must meet certain requirements as defined by various standards set by organizations such as the SAE, or local government equivalents. For example, most brake fluid sold in North America is classified by the US Department of Transportation
Department of Transportation
The Department of Transportation is the most common name for a government agency in North America devoted to transportation. The largest is the United States Department of Transportation, which oversees interstate travel. All U.S. states, Canadian provinces, and many local agencies also have...
(DOT) under their own ratings such as "DOT 3
DOT 3
DOT 3 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point....
" and "DOT 4
DOT 4
DOT 4 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point....
". Their classifications broadly reflect the concerns addressed by the SAE's specifications, but with local details - Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
and the Azores
Azores
The Archipelago of the Azores is composed of nine volcanic islands situated in the middle of the North Atlantic Ocean, and is located about west from Lisbon and about east from the east coast of North America. The islands, and their economic exclusion zone, form the Autonomous Region of the...
have different normal temperature and humidity ranges to consider, for example. Many countries defer explicitly to the SAE specifications, or simply refer to "best practice" which in practice would defer to the SAE.
Characteristics
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.Boiling point
Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization is a problem because vapor is compressible and negates hydraulic fluid transfer of braking force.Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent, varying from formulation to formulation. Glycol-ether/dot three/dot four/dot five point one brake fluids are hygroscopic
Hygroscopy
Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed,' somewhat, by an increase in volume, stickiness, or other...
(water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
/DOT 5
DOT 5
DOT 5 is one of several designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.DOT 5 is a silicone-based brake fluid ....
-based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life, although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.
| Dry boiling point | Wet boiling point | |
|---|---|---|
| DOT 3 DOT 3 DOT 3 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.... |
205 °C (401 °F) | 140 °C (284 °F) |
| DOT 4 DOT 4 DOT 4 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.... |
230 °C (446 °F) | 155 °C (311 °F) |
| DOT 5 DOT 5 DOT 5 is one of several designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.DOT 5 is a silicone-based brake fluid .... |
260 °C (500 °F) | 180 °C (356 °F) |
| DOT 5.1 DOT 5.1 DOT 5.1 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.... |
270 °C (518 °F) | 190 °C (374 °F) |
Wet boiling point defined as 3.7% water by volume.
Viscosity
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 an anti-lock braking systemAnti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...
(ABS), traction control, and stability control (ESP).
Corrosion
Brakes fluids must not corrode the metals used inside components such as calipers, master cylinders, etc. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this.Compressibility
Brake fluids must maintain a low level of compressibility that remains low, even with varying temperatures..Service and maintenance
Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed, or changed, every 1–2 years. Many manufacturers also require periodic fluid changes to ensure reliability and safety. Once installed, moistureMoisture
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...
diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high. Electronic testers and test strips are commercially available to measure moisture content. The corrosion inhibitors also degrade over time. New fluid should always be stored in a sealed container to avoid moisture intrusion.
DOT 5 is silicone fluid and the above does not apply. Ideally, silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol based fluid will contain moisture; glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climate, particularly in Russia and Finland.
A small drop in brake fluid level in the master cylinder reservoir can be "topped up" but if the level consistently drops, the cause should be investigated and repaired. Brake fluid level in the master cylinder will drop as the linings (pads or shoes) wear and the calipers or wheel cylinders extend further to compensate. Overspill from pushing back pistons should be avoided, because glycol based fluid will quickly lift or strip paints and other coatings on contact (it can be removed by quickly washing with water, not wiping). Brake fluid level may also be low because of a leak, which could result in a loss of hydraulic pressure and consequently a significant loss of braking ability. Modern cars have split hydraulic circuits to ensure against total hydraulic failure. Brake fluids with different DOT ratings should not be mixed; not all DOT fluid is compatible. This is because it will dilute and reduce the properties of the higher specification DOT fluid, or in the case of mixing of glycol with silicone fluid may cause corrosion due to trapped moisture.
Brake fluid can be dangerous as it is toxic and flammable but has a high flash point. It can become explosive in the presence of Chlorine powder and acts as a mild paint remover if left on painted surfaces.
Glycol-based (DOT 3, 4, 5.1)
- Alkyl ester
- Aliphatic amineAmineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
- Diethylene glycolDiethylene glycolDiethylene glycol is an organic compound with the formula 2O. It is a colorless, practically odorless, poisonous, and hygroscopic liquid with a sweetish taste. It is miscible in water, alcohol, ether, acetone, and ethylene glycol. DEG is a widely used solvent...
- 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 sebacateDioctyl sebacateDioctyl sebacate 82, or di sebacate, is an organic compound which is the diester of sebacic acid and 2-ethylhexanol. It is an oily colorless liquid and is used as a plasticizer. Most commonly in C4. It has also found use in Dot 5 brake fluid.-Physical properties:Relative vapour density :...
- Dimethyl polysiloxane
- Tributyl phosphateTributyl phosphateTributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...