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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. 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 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.
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 compressible and negates hydraulic fluid transfer of braking force.
Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Oftentimes, a car will become flooded with brake fluid. 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.
|Dry boiling point||Wet boiling point|
|DOT 3||205 °C (401 °F)||140 °C (284 °F)|
|DOT 4||230 °C (446 °F)||155 °C (311 °F)|
|DOT 5||260 °C (500 °F)||180 °C (356 °F)|
|DOT 5.1||270 °C (518 °F)||190 °C (374 °F)|
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).
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.
Brake fluids must maintain a low level of compressibility that remains low, even with varying temperatures..
Service and maintenanceEdit
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, 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. 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, particularily 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)Edit
- 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)Edit
- What happens when the corrosion inhibitors in brake fluid become depleted
- Car care council
- How Stuff Works: What are the different types of brake fluid?
- StopTech: Brake Fluid 1A
- Brake Bleeding
- All brake fluid information
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