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.

The origins of modern braking systems date back to an event more than a century ago - in 1917, Scotsman Malcolm Lockheed patented a hydraulic actuated braking system. Initially, vegetable oil was used as a working fluid. But it did not meet the most basic requirements, and in the process of evolution, special brake fluids were created, which consist of a base and a package of additives (thickeners, anti-corrosion additives, colorants).

Standards
Most Brake fluids are manufactured to meet standards set by international, national, or local organizations or government agencies.

International
The International Standards Organisation has published its standard ISO 4925, defining classes 3, 4, and 5, as well as class 5.1, class 6 and class 7  reflecting progressively higher performance for brake fluids.

SAE
The Society of Automotive Engineers SAE has published standards J1703, J1704, and J1705, reflecting progressively higher performance for brake fluids. These have counterparts in the international standard, ISO 4925.

United States
The Federal Motor Vehicle Safety Standards (FMVSS) under FMVSS Standard No. 116 defines grades DOT 3, DOT 4, DOT 5 and DOT 5.1, where DOT refers to the U.S. Department of Transportation. These are widely used in other countries. Their classifications broadly reflect the SAE's specifications, DOT 3 is equivalent to SAE J1703 and ISO class 3, DOT 4 to SAE J1704 and ISO class 4, etc.

All DOT compliant fluids must be colorless or amber, except for DOT 5 silicone, which must be purple. FMVSS Standard No. 116's scope is limited to fluid 'for use'. Brake fluid 'in use', or not labeled DOT compliant, is found in any color.

DOT 4 
While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 (a temperature upgrade) if the elastomers in the system accept the borate compounds that raise the boiling point, a vehicle that requires DOT 4 might boil the brake fluid if a DOT 3 (a temperature downgrade) is used. Additionally, these polyglycol-ether-based fluids cannot be mixed with DOT 5.0, which is silicone based.

DOT 5 
DOT 5 is a silicone-based fluid and is separate from the series of DOT 2, 3, 4, 5.1. It is immiscible with water, and with other brake fluids, and must not be mixed with them. Systems can change fluid only after a complete system changeover, such as a total restoration.

It contains at least 70% by weight of a diorgano polysiloxane. Unlike polyethylene glycol based fluids, DOT 5 is hydrophobic. An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range. Another property is that it does not damage paint.

DOT 5 brake fluid is not compatible with anti-lock braking systems. DOT 5 fluid can aerate when the anti-lock brake system is activated. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air.

DOT 5.1 
Lack of acceptance of silicone-based fluids led to the development of DOT 5.1, a fluid giving the performance advantages of silicone, whilst retaining some familiarity and compatibility with the glycol ether fluids. DOT 5.1 is the non-silicone version of DOT 5, defined by FMVSS 116 as being less than 70% silicone. Above that threshold makes it DOT 5.

Characteristics
Brake fluids must have certain characteristics and meet certain quality standards for the braking system to work properly.

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 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 a faster reaction of the ABS and ESP systems, DOT 4 and DOT 5.1 brake fluids exist with low viscosity meeting the maximum 750 /s viscosity at  requirement of ISO 4925 class 6. These are often named DOT 4+ or Super DOT 4 and DOT 5.1 ESP.

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.

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 5 and 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. 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.

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 than glycol-ether based DOT fluids.

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 with the rubber.

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.

Functions 
When the driver depresses the brake pedal, pressure is transmitted to the brake master cylinder. The brake cylinder piston pressurizes a system of hydraulic tubes, each of which leads to a different wheel. The brake fluid in the tubes, in turn, pressurizes the brake slave cylinders, which are on each wheel. The slave cylinder pistons press down the brake pads. They encompass and compress the brake disk, and the rotation of the wheels slows down. 

In addition to transmitting pressure, brake fluid also keeps the brake system working optimally. It helps to regulate temperature, ensuring that components are resistant to the heat generated during braking. Proper maintenance of the brake fluid level is critical, as low levels or contaminated fluid can lead to reduced braking performance and, in extreme cases, brake failure.

Brake fluid is mainly used on brake systems, but is also widely used for hydraulically controlled clutches. 

Depending on the application, the fluid is subjected to different pressures: in the case of motorcycles, it has pressure peaks that range from 8 to 15 bar, while in Formula 1 cars it exceeds 75 bar.

Service and maintenance

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. 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 (DOT 3/4/5.1) 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.

Brake fluids with different DOT ratings can not always be mixed. DOT 5 should not be mixed with any of the others as mixing of glycol with silicone fluid may cause corrosion because of trapped moisture. DOT 2 should not be mixed with any of the others. 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 fluids with fresh to obtain the specified performance.

Brake fluid is toxic and can damage painted surfaces.

In the vast majority of cars - those fueled by glycol compounds - brake fluid needs to be changed periodically. Generally, operating instructions require this to be done every two years. Do not neglect this service operation, because the boiling point of the liquid steadily decreases over time. And it will boil not gradually, but suddenly - and the car will be without brakes at one moment. Maximum inappropriate for this. 

Another signal for a complete fluid change is any darkening of the fluid or lack of transparency. You should react especially quickly if you find black deposits in the expansion tank. This is already a sign of wear of cuffs or even pistons in the master cylinder, fraught with brake failure. 

If after topping up the brake fluid, its level has dropped, it already indicates a leaky system and the need to urgently search for and eliminate the leakage. It is dangerous to operate such a car, because a small hole can become a big tear in just one pedal stroke - and then the car will be without brakes at all. The consequences of such a refusal can already be fatal.

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

See also
 Brake fluid pressure sensor
 Hydropneumatic suspension
 Pascal's law

References

External links 
 Car care council
 How Stuff Works: What are the different types of brake fluid?
 StopTech: Brake Fluid 1A 

Vehicle braking technologies
Automotive chemicals
Hydraulic fluids