Brake fluid is a special type of hydraulic fluid used in brake systems. Most brake fluids today are based on synthetic components with added corrosion inhibitors.
Brake fluids are used in the hydraulic brake systems of passenger cars and light commercial vehicles to transmit pressure to the brake mechanisms. Another field of application is in hydraulic clutch systems. They are used also in the brake systems of motorcycles.
As the brake system operates, a large amount of heat develops and for this reason the brake fluid must have a high boiling point to be able to function effectively (dry and wet boiling point). Brake fluids must not freeze at negative weather temperatures. These requirements exclude the option of using water-based brake fluids.
- High boiling temperature;
- The correct viscosity and good ability to flow at low temperatures;
- Resistance to aging;
- Wide operating temperature range and low volatility;
- Excellent compatibility with sealing materials.
The basic properties of brake fluids are regulated by the following specifications:
- SAE J1702, J1703, J1704
- ISO 4925 Class X; X=3,4…..
- FMVSS №116 – US Federal Motor Vehicle Safety Standard;
- JIS K2233 Class X; X=3,4 – Japan Industrial Standards;
- ATE-DOT X; X=3,4,5.1– Operational Applications Standard;
Formerly used specifications:
- SAE 70 R 1 and 3; SAE J70C; SAE J 1703 А until F and SAE J 1703 Jan.’80, Nov.’83 and Oct.’85.
DOT has become a common name for brake fluid. The abbreviation itself means the US Department оf Transportation (DOT).
Brake fluid exists in different forms meeting the US Department of Transportation specifications.
- These specifications are DOT 3, DOT 4 and DOT 5.1
- DOT 5 can also be met in some applications but it differs from the above specifications in its chemical composition. This form of brake fluid is practically unusable in conventional vehicles.
DOT 3, DOT 4 and DOT 5.1 are polyethylene glycol-based fluids in contrast with DOT 5 which is silicone-based.
DOT 3, DOT 4 and DOT 5.1 are hygroscopic and will absorb water and moisture from the atmosphere. Water absorption degrades the performance characteristics of the fluid and drastically reduces its boiling point (which is one of the reasons necessitating its change).
Glycol-based fluids are:
- Twice as less compressible as silicone fluids, even when heated;
- The lower compressibility of the fluid leads to a better system performance and increases the feeling of brake pedal firmness;
Storage: glycols are hygroscopic and absorb water from the atmosphere which leads to:
- Lower boiling point of the brake fluid
- Weakened hydraulic efficiency.
The water content increases the viscosity at low temperatures and intensifies corrosion activity.
DOT 5 is a silicone fluid, incompatible with other polyethylene glycol-based fluids. The main difference is that silicone fluid is hydrophobic.
Brake fluid change will ensure a normal and safe brake system operation. On the other hand, its replacement on a regular basis will extend the life of brake components.
KEY ISSUES in handling brake fluid:
- This kind of fluid is toxic and flammable and should be handled with the necessary care and precautions;
- Brake fluid should not be poured into the hydro-pneumatic system of the car (this system is filled with other products, such as LHM and the like);
- Brake fluid storage containers should be perfectly clean and hermetically sealed at all time to prevent moisture intrusion from the air.
- Brake fluid ingredients have a strong corrosive effect on car paint, lacquer and chromium-plated parts which must be taken into consideration when handling the product.
- DOT 3 can be mixed with DOT 4
- DOT 3 and DOT 4 can also be mixed with brake fluids of the DOT 5.1 rating;
- DOT 5(silicone based) IS INCOMPATIBLE with other DOT fluids
For topping up, any commercial brand can be used as long as technical requirements are met. A lower-specification fluid may be replaced with a higher DOT-rating fluid (e.g. DOT3 can be replaced with DOT4).
In a car system filled with DOT5 none of the other ratings of brake fluid, i.e. DOT 3, DOT 4 or DOT 5.1) should be added.
Topping-up cannot compensate the irreversible changes taking place in the ageing fluid. The boiling point drops. The chemical composition is changed. The corrosion inhibitors degrade over time.
Specialized car service centres provide diagnostics with testing equipment. If no information is available as to when and whether the brake fluid has been changed, either change it without further delay, or have the brake system checked.
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- Widespread commercial distribution;
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- Professional approach in our branded auto centres throughout the country.
The recommended brake fluid change interval is every two years!