Brake Fluid: What It Is and Its Designations
Brake fluid is a vital part of any vehicle equipped with a hydraulic braking system. It facilitates the transfer of the force needed to slow down or stop the vehicle. Choosing the wrong type or neglecting maintenance can significantly affect driving safety.
This article will cover what brake fluid is, the different types available, how to use them correctly, and what to be cautious about.
Content
What Is the Purpose of Brake Fluid?
The primary function of brake fluid is to transmit force through pressure. Liquids are not easily compressible, so the force applied is directly transmitted by compressing the chemical bonds within the fluid.
Since oils can damage the seals and hoses in the braking system, brake fluids are generally not made from petroleum. Most brake fluids are formulated from glycol and ether, although some are based on mineral oils.
Braking System Malfunctions: How do they manifest?
All brake fluids must meet specific requirements set by various organizations, such as the Society of Automotive Engineers (SAE) and regulations from different authorities.
The acronym DOT, which stands for the American Department of Transportation, is used globally to designate brake fluid and its performance specifications.
Brake Fluid Designations
DOT 1
DOT 2
DOT 3
DOT 4
DOT 5
DOT 5.1
DOT 2, DOT 3, and DOT 4 fluids are generally compatible with one another, but DOT 5 cannot be mixed or substituted due to its unique composition. DOT 5 is silicone-based, while the others are polyethylene glycol-based. DOT 5.1 is based on the same formulation as DOT 4 and is suitable for extreme conditions.
Because DOT 5.1 uses a polyethylene glycol base, it can be mixed with DOT 4 fluid. As a rule of thumb, brake fluids with a higher designation number tend to be of better quality and more suitable for demanding conditions than those with a lower number.
Designation | Dry Boiling Point | Wet Boiling Point | Viscosity | Main Component |
DOT 2 | 190 °C | 140 °C | - | Castor Oil/Alcohol |
DOT 3 | 205 °C | 150 °C | 1500 mm2/s | Glycol Ether |
DOT 4 | 230 °C | 155 °C | 1800 mm2/s | Glycol Ether |
LHM | 249 °C | 249 °C | 1200 mm2/s | Mineral Oil |
DOT 5 | 260 °C | 180 °C | 900 mm2/s | Silicone |
DOT 5.1 | 260 °C | 180 °C | 900 mm2/s | Glycol Ether |
The dry boiling point is the temperature at which new fluid, free from moisture, begins to boil, while the wet boiling point refers to fluid that has absorbed 3.7% water—the average moisture absorption value.
1. Viscosity
For optimal braking system performance, brake fluid must maintain consistent viscosity across a wide range of temperatures, even in extremely low conditions.
Anti-lock braking system (ABS)
This is especially crucial for braking systems that feature an ABS (Anti-lock Braking System) or an ESP (Electronic Stability Program).
2. Compressibility
Brake fluids must exhibit low compressibility, regardless of temperature fluctuations. This characteristic is vital for ensuring a consistent response from the brake pedal.
If brake fluid lacks low compressibility, the brake pedal will need to travel farther to transmit the same amount of force to the brake caliper piston.
3. Corrosion
Brake fluids must not cause corrosion of the metals used in the braking system components and should protect these parts from corrosion when moisture gets into the system.
A key advantage of mineral oil-based brake fluid (LHM) is its resistance to corrosion.
Conclusion
Choosing the right brake fluid and maintaining it properly are essential for your vehicle's safety. Always follow the manufacturer's recommendations and regularly check the fluid's condition to minimize the risk of brake failure.