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.

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.[1]

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 its 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 for example, have different normal temperature and humidity ranges to consider. Many countries[weasel words] defer explicitly to the SAE specifications, or simply refer to “best practice” which in application would defer to SAE standard. 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.

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 creates a problem because vapor is highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force – so the brakes will fail to stop the vehicle.[2]

Quality standards refer to a brake fluid’s “dry” and “wet” boiling points. The 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 3, 4, and 5.1) 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 5and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid’s service life.

Silicone based fluid is more compressible than glycol based fluid, leading to brakes with a spongy feeling.[3] It can potentially suffer phase separation/water pooling and freezing/boiling in the system over time – the main reason single phase hygroscopic fluids are used.

Characteristics of common braking fluids[4][5]
Dry boiling point Wet boiling point Viscosity limit Primary constituent
DOT 2 190 °C (374 °F) 140 °C (284 °F) ? castor oil/alcohol
DOT 3 205 °C (401 °F) 140 °C (284 °F) 1500 mm2/s glycol ether
DOT 4 230 °C (446 °F) 155 °C (311 °F) 1800 mm2/s glycol ether/borate ester
LHM+ 249 °C (480 °F) 249 °C (480 °F) 1200 mm2/s [6] mineral oil
DOT 5 260 °C (500 °F) 180 °C (356 °F) 900 mm2/s silicone
DOT 5.1 260 °C (500 °F) 180 °C (356 °F) 900 mm2/s glycol ether/borate ester

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 system (ABS), traction control, and stability control (ESP), as these systems may use a valve with a time-based approach, rather than measuring pressure or volume to control the amount of fluid transferred.

Corrosion

Brake fluids must not corrode the metals used inside components such as calipers, wheel cylinders, master cylinders and ABS control valves. They must also protect against corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to the base fluid to accomplish this. Silicone is less corrosive to paintwork unlike glycol-ether based DOT fluids.[7]

The advantage of the Citroën LHM mineral oil based brake fluid is the absence of corrosion. Seals may wear out at high mileages but otherwise these systems have exceptional longevity. It cannot be used as a substitute without changing seals due to incompatibility.[8][not in citation given]

Compressibility

Brake fluids must maintain a low level of compressibility, 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 necessary for the same amount of brake caliper piston force.

Service and maintenance

“600 m ahead, a 38-km long continuous descent starts. Please check your brakes and add brake cooling water!” A warning sign on a highway in Yunnan, where it drops ca. 1,500 m over a 38 km distance. Water is sprayed or dripped on brake drums for cooling.

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 under non-racing conditions.[9] Many manufacturers also require periodic fluid changes to ensure reliability and safety. Once installed, moisture 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, however moisture test strips were taken off the market because they absorb moisture in the air before they can be used. The corrosion inhibitors also degrade over time. Degraded inhibitors allow corrosion in the braking system. The first metal to corrode is copper. You can determine when it is time to replace brake fluid when copper ions hit 200ppm.[10] 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 climates, 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 redundant hydraulic circuits (two separate circuits) to ensure against total hydraulic failure.

Brake fluids with different DOT ratings can not always be mixed. DOT 5 should not be mixed with any of these as mixing of glycol with silicone fluid may cause corrosion because of trapped moisture.

Brake fluid is toxic[11] and can damage painted surfaces.[12]

Components

Castor oil-based (pre-DOT, DOT 2)

  • Castor oil
  • Alcohol, usually butanol (red / 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

References

  1.  http://www.peterverdone.com/archive/files/hydraulic%20system%20theory.pdf
  2.  http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/
  3.  http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/
  4.  “49 CFR 571.116 – Standard No. 116; Motor vehicle brake fluids”.
  5.  http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/
  6.  http://www.viscopedia.com/viscosity-tables/substances/automotive-brake-fluid/
  7.  http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/
  8.  http://www.peterverdone.com/archive/files/hydraulic%20system%20theory.pdf
  9.  Bosch Automotive Handbook Archived October 27, 2008, at the Wayback Machine., 7th Edition, ISBN 978-0-7680-1953-7
  10.  [1]
  11.  “MSDS for DOT 3 brake fluid” (PDF). Retrieved 2012-06-04.
  12.  [2]
Transmission brake A transmission brake or driveline parking brake is an inboard vehicle brake that is applied to the drivetrain rather than to the wheels. Historically, some early cars used transmission brakes as the normal driving brake and often had wheel brakes on only one axle, if that. In current vehicles, these brakes are now rare. They are found in some makes, notably Land Rover, usually for light off-road vehicles. Simple transmission brakes could be found in large vehicles too, such as the 16 inch single disc parking brake used in the M19 Tank Transporter of World War II. The transmission brake is provided solely as a parking brake or handbrake. Normal wheel brakes are still provided for use when driving, drum brakes originally, now almost always disc brakes. Driver's manuals usually caution against using the transmission brake when driving, as it is neither powerful enough nor robust enough and so will not work effectively and may even be damaged by trying to stop a moving vehicle. ...
Automatic transmission fluid Automatic transmission fluid (ATF), also known as transmission fluid or tranny fluid for short, is the fluid used in vehicles with self shifting or automatic transmissions. It is typically coloured red or green to distinguish it from motor oil and other fluids in the vehicle. The fluid is optimized for the special requirements of a transmission, such as valve operation, brake band friction and the torque converter as well as gear lubrication. ATF is also used as a hydraulic fluid in some power assisted steering systems, as a lubricant in some 4WD transfer cases, and in some modern manual transmissions. Automatic transmission fluid Modern use Modern ATF typically contains a wide variety of chemical compounds intended to provide the required properties of a particular ATF specification. Most ATFs contain some combination of additives that improve lubricating qualities, such as anti-wear additives, rust and corrosion inhibitors, detergents, dispersants and surfactants (whic...
Disc brake A disc brake is a type of brake that uses calipers to squeeze pairs of pads against a disc or "rotor" to create friction. This action retards the rotation of a shaft, such as a vehicleaxle, either to reduce its rotational speed or to hold it stationary. The energy of motion is converted into waste heat which must be dispersed. Hydraulically actuated disc brakes are the most commonly used form of brake for motor vehicles, but the principles of a disc brake are applicable to almost any rotating shaft. Design On automobiles, disc brakes are often located within the wheel A drilled motorcycle brake disc Development of disc-type brakes began in England in the 1890s. In 1902, the Lanchester Motor Company designed brakes that looked and operated in a similar way to a modern disc-brake system even though the disc was thin and a cable activated the brake pad.Other designs were not practical or widely available in cars for another 60 years. Successful...
Brake pad Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are steel backing plates with friction material bound to the surface that faces the disc brake rotor. Function Brake pads convert the kinetic energy of the vehicle to thermal energy through friction. Two brake pads are contained in the brake caliper, with their friction surfaces facing the rotor. When the brakes are hydraulically applied, the caliper clamps or squeezes the two pads together onto the spinning rotor to slow/stop the vehicle. When a brake pad heats up due to contact with the rotor, it transfers small amounts of its friction material onto the disc, leaving a dull grey coating on it. The brake pad and disc (both now having the friction material), then "stick" to each other, providing the friction that stops the vehicle. In disc brakes, there are usually two brake pads per disc rotor. These are held in place and actuated by a caliper affixed to the wheel hub or suspension upr...
Biodiesel Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl (methyl, ethyl, or propyl) esters. Biodiesel is typically made by chemically reacting lipids (e.g., vegetable oil, soybean oil, animal fat (tallow)) with an alcohol producing fatty acid esters. Biodiesel is meant to be used in standard diesel engines and is thus distinct from the vegetable and waste oils used to fuel converted diesel engines. Biodiesel can be used alone, or blended with petrodiesel in any proportions. Biodiesel blends can also be used as heating oil. The National Biodiesel Board (USA) also has a technical definition of "biodiesel" as a mono-alkyl ester. Space-filling model of methyl linoleate, or linoleic acid methyl ester, a common methyl ester produced from soybean or canola oil and methanol Space-filling model of ethyl stearate, or stearic acid ethyl ester, an ethyl ester produced from soybean or canola oil and ethanol ...