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 — 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 compress the fluid's chemical bonds.
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 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 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.
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.
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 highly compressible relative to liquid, and therefore negates the hydraulic transfer of braking force - so the brakes will fail to stop the car. 
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 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 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life.
Silicon based fluid is more compressible than glycol based fluid, leading to spongy feeling brakes.  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.
|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 ||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.
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.
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.
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.
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
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. 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 cause 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. 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.
The "high" and "low" lines on the brake fluid reservoir are there for a quick visual reference to check the condition of brake pads/rotors without having to remove the wheel. On a new car from the factory, with brand new rotors and pads, the brake fluid will be at the "high" mark, and as the pads and rotors get worn over time the fluid level in the reservoir drops, not because the fluid was "used", rather it drops because the pistons in the calipers are extended further at rest than they would be with new pads and rotors. So when the brake fluid is at the "low" mark it is time to have the brakes inspected and serviced.
And if new pads are installed and the existing rotors "turned" to refinish the braking surface, the fluid reservoir may not be at the "full" line and that is normal because of two things.
The first reason why the fluid level may not be "full" after installation of new brake pads and with rotors refinished (turned down) is that the rotor has been worn down and then machined down to give the pads a new smooth finish to bite into, and some cars – particularly cars made in the year 2000 – use steel rotors and the "nominal thickness" can be as much as 0.125" thinner than a new rotor. This small 0.125" discrepancy can add up; for example, for 4 wheels, the total discrepancy adds up to 0.5", which translates to a significant height difference of brake fluid as it appears in the reservoir. With used brake pads and rotors the pistons are not as compressed because the rotors and pads are thinner than they were when new, and there is more fluid in the pistons inside the caliper and therefore less in the reservoir.
The second reason why the brake fluid reservoir may not be "full" even after getting all 4 rotors and all new pads installed, is that aftermarket parts and even sometimes original manufacturer parts have been slightly changed, and are built to slightly different thicknesses than the factory originals. It is perfectly safe but if the brake fluid is not flushed out or if the calipers or wheel cylinders are not replaced the mechanic has no reason to "top off" the system, especially since when it is "topped off" the next time someone does replace the pads and rotors, the pistons must be fully compressed so the new pads can fit over the new rotors, and if new fluid has been previously added, the system will simply overflow from the reservoir and make a huge mess. Brake fluid does strip paint pretty easily and could damage the laminate in a windshield because the brake fluid may be sprayed all over the place after being forced past the reservoir cover. It is a huge hassle and can and does happen.
Brake fluids with different DOT ratings can not always be mixed. It must be of the same type, and at least the same rating. DOT 5.1 can replace DOT 4 and 3, DOT 4 can replace DOT 3. DOT 5 should not be mixed with any of these as mixing of glycol with silicone fluid may cause corrosion because of trapped moisture.
Castor oil-based (pre-DOT, DOT 2)
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)
- "Standard No. 116; Motor Vehicle Brake Fluids". U.S. Department of Transportation. 12 Apr 2013. Retrieved 22 May 2013.
- Bosch Automotive Handbook, 7th Edition, ISBN 978-0-7680-1953-7
- "MSDS for DOT 3 brake fluid" (PDF). Retrieved 2012-06-04.
- Car care council
- How Stuff Works: What are the different types of brake fluid?
- StopTech: Brake Fluid 1A