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Brake fluid is the hydraulic medium employed to transmit actuation forces within the braking system. Compliance with stringent requirements is essential to ensure reliable brake system operation. This requirements are defined in various standards of similar content (SAE J 1703, FMVSS 116, ISO 4925). The performance data contained in FMVSS 116 (Federal Motor Vehicle Safety Standard), mandatory in the United States, serves as an international reference.
The equilibrium boiling point provides an index of the brake fluid's resistance to thermal stress. The heat encountered in the wheel cylinders (with the highest braking temperatures in the entire braking system) is especially critical. Vapor bubbles can form at the temperatures above the brake fluids instantaneous boiling point, resulting in catastrophic brake failure.
The wet boiling point is the fluids equilibrium boiling point subsequent to moisture adsorption under specified conditions (approximately 3.5%). Hydroscopic (glycol based) fluids respond with an especially pronounced drop in boiling point when contaminated with water.
DOT SPECIFICATIONS
DOT 3 is polyethelene glycol and is amber colored
DOT 4 is polyethelene glycol with esters and is amber colored
DOT 5 is silicone and purple colored to prevent its accidental use
DOT 5.1 is polyethelene glycol with esters and is amber colored
DOT 3 - dry boiling point = 205C = 450F
wet boiling point 140C = 284F
DOT 4 - dry boiling point 230C 513F
wet boiling point 155C = 333F
DOT 5 - dry boiling point 260C
wet boiling point 180C
DOT 5.1 - don't have exact specifications but it is used in extremely high performance motorcycle braking systems like the Brembo system on the KTM 640 Adventure Rally race bike! Also used in Formula 1 exotic race cars as well.
ALL VOLKSWAGEN TDI'S SHOULD USE DOT 4 BRAKE FLUID ONLY
Most brake fluids are glycol ether compounds. These generally consist of monoethers of low polyethelene glycols and are therefore polyalkylene glycol ether compounds. Although these compounds may be used to produce a brake fluid which conforms to DOT 3 requirements, their undesirable hygroscopic properties cause these fluids to absorb moisture at a relatively rapid rate with an attendant swift reduction in the boiling point.
If the free OH (hydroxyl) groups are partially esterified with boric acid, the result is superior DOT 4 brake fluid capable of reacting with water and neutralizing its effects. Also, the DOT 4 brake fluid's boiling point drops much more slowly than that of DOT 3 when contaminated with water, increasing the service life of DOT 4 greater than DOT 3. DOT 4 is superior to DOT 3 in every possible manner and is much preferred for brake fluid in modern automobiles.
There are two other brake fluids that are used for racing and are not approved for road use, they are mineral-oil fluids and silicone fluids. DOT 5 is silicone brake fluid. These great advantage of these fluids are that they are not hygroscopic, so the boiling point does not drop owing to moisture adsorption. The disadvantage of these products is that they have higher compressibility and inferior lubrication, which reduces their general use. Also, they should NEVER be mixed with brake fluids containing glycol ethers (or vice versa) as this destroys the elastomers in the fluids altering the relationship between temperature and viscosity detrimentally.
Remember, DO NOT SPILL Brake Fluid on your paint, it WILL dissolve your cars paint! Also, do not spill it on radiator hoses either.
The equilibrium boiling point provides an index of the brake fluid's resistance to thermal stress. The heat encountered in the wheel cylinders (with the highest braking temperatures in the entire braking system) is especially critical. Vapor bubbles can form at the temperatures above the brake fluids instantaneous boiling point, resulting in catastrophic brake failure.
The wet boiling point is the fluids equilibrium boiling point subsequent to moisture adsorption under specified conditions (approximately 3.5%). Hydroscopic (glycol based) fluids respond with an especially pronounced drop in boiling point when contaminated with water.
DOT SPECIFICATIONS
DOT 3 is polyethelene glycol and is amber colored
DOT 4 is polyethelene glycol with esters and is amber colored
DOT 5 is silicone and purple colored to prevent its accidental use
DOT 5.1 is polyethelene glycol with esters and is amber colored
DOT 3 - dry boiling point = 205C = 450F
wet boiling point 140C = 284F
DOT 4 - dry boiling point 230C 513F
wet boiling point 155C = 333F
DOT 5 - dry boiling point 260C
wet boiling point 180C
DOT 5.1 - don't have exact specifications but it is used in extremely high performance motorcycle braking systems like the Brembo system on the KTM 640 Adventure Rally race bike! Also used in Formula 1 exotic race cars as well.
ALL VOLKSWAGEN TDI'S SHOULD USE DOT 4 BRAKE FLUID ONLY
Most brake fluids are glycol ether compounds. These generally consist of monoethers of low polyethelene glycols and are therefore polyalkylene glycol ether compounds. Although these compounds may be used to produce a brake fluid which conforms to DOT 3 requirements, their undesirable hygroscopic properties cause these fluids to absorb moisture at a relatively rapid rate with an attendant swift reduction in the boiling point.
If the free OH (hydroxyl) groups are partially esterified with boric acid, the result is superior DOT 4 brake fluid capable of reacting with water and neutralizing its effects. Also, the DOT 4 brake fluid's boiling point drops much more slowly than that of DOT 3 when contaminated with water, increasing the service life of DOT 4 greater than DOT 3. DOT 4 is superior to DOT 3 in every possible manner and is much preferred for brake fluid in modern automobiles.
There are two other brake fluids that are used for racing and are not approved for road use, they are mineral-oil fluids and silicone fluids. DOT 5 is silicone brake fluid. These great advantage of these fluids are that they are not hygroscopic, so the boiling point does not drop owing to moisture adsorption. The disadvantage of these products is that they have higher compressibility and inferior lubrication, which reduces their general use. Also, they should NEVER be mixed with brake fluids containing glycol ethers (or vice versa) as this destroys the elastomers in the fluids altering the relationship between temperature and viscosity detrimentally.
Remember, DO NOT SPILL Brake Fluid on your paint, it WILL dissolve your cars paint! Also, do not spill it on radiator hoses either.