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Brake fluid is a type of hydraulic fluid used in hydraulic brake applications in motorcycles, automobiles, light trucks, and some advanced bicycles. It is used to transfer force under pressure from where it is created through hydraulic lines to the braking mechanism near the wheels. It works because liquids are not appreciably compressible. Brake fluid is also commonly used in hydraulic clutches.

Most brake fluids used today are glycol-ether based, but mineral oil and silicone based fluids are also available. Brake fluids must meet certain requirements as defined by various standards set by organizations such as the SAE. Most brake fluid sold in North America is organized by the US Department of Transportation (DOT) rating. "DOT 3" and "DOT 4" are examples.

Contents 1 Characteristics 1.1 Boiling point 1.2 Viscosity 1.3 Corrosion 1.4 Compressibility 2 Service and maintenance 3 Components 3.1 Mineral-based 3.2 Silicone-based 4 References 5 See also 6 External links

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 released in to the lines is compressible and would result in an inability of the hydraulic fluid to transfer braking force. Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet boiling point, which is usually much lower, refers to the fluid's boiling point after absorbing a certain amount of moisture. This amount is 3.7%, which is considered the safe limit for moisture content. Glycol-ether brake fluids are hygroscopic (water loving), which means they absorb moisture. Modern fluids must be able to maintain an acceptable boiling point as they absorb moisture over the fluids service life.

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 Antilock brakes (ABS), Traction Control, and Stability Control.

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 low level of compressibility that remains low, even with varying temperatures.

Service and maintenance

Most automotive professionals agree that brake fluid should be flushed, or changed, every 1-2 years.¹ 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.

Brake fluid is not considered a "top up" fluid. If it is low, there is usually a problem. 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. Brake fluid level may also be low because of a leak, which could result in a loss of hydraulic pressure and consequently, a loss of braking ability. As a general rule, brake fluids with different DOT ratings should not be mixed.

Brake fluid can be dangerous as it is toxic and highly flamable. It will also lift or strip paints and other coatings on contact.

Components

Mineral-based

• 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

• Di-2-ethylhexyl sebacate
• Dimethyl polysiloxane
• Tributyl phosphate

References

See also

• Disc brake
• Drum brake
• Brake bleeding
• Hydraulic brake
• Hydropneumatic

External links

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

Wikipedia content modification information:

• This page was last modified on 4 December 2008, at 02:10.

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