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Home >   How To >   How to Repair Brake System >   What is Brake Fluid?

What is Brake Fluid?



Brake fluid is a critical aspect of the braking system, that supplies the required hydraulic pressure for efficient braking. It is also responsible for lubricating the brake components. The brake fluid is basically an incompressible fluid which has a high boiling point. It is also designed to meet a specific viscosity limit. The force applied on the brake pedal is transformed into the requisite brake pressure and carried through the brake master cylinder. This pressure is then applied on the calipers to squeeze the brake pads against the disc. The brake fluid should meet the regulations of organizations such as the SAE International or other equivalent firms.





Types of Brake Fluids


There are four main classifications of brake fluids: DOT 3, DOT 4, DOT 5 and DOT 5.1.


DOT 3: The DOT 3 is a glycol-based fluid. It is used in most cars and light trucks. The DOT 3 has a minimum dry boiling point at 205ºC and a minimum wet boiling point at 140ºC.


DOT 4: These type of fluids are generally employed in large cars and trucks. They are also commonly used in automobiles with ABS (Anti-lock Braking Systems). DOT 4 is also a glycol-based fluid. The fluid has a minimum dry boiling point at 230ºC and a minimum wet boiling point at 155ºC.


Note: DOT 3 and DOT 4 fluids can absorb moisture, due to which their boiling point decreases. That is why it is recommended to not expose a brake fluid reservoir with these fluids to the atmosphere. For the same reason, containers of these brake fluids are also tightly sealed.


DOT 5: The DOT 5 is a silicon-based fluid. It is generally used in antique cars and collector vehicles. It is not recommended for use in ABS systems. The DOT 5 fluid does not absorb water and therefore, its boiling point remains relatively stable. However, any water in the system will form water pockets, which could cause the brake parts to corrode. The DOT 5 has a minimum dry boiling point at 260ºC while its minimum wet boiling point is at 180ºC.


DOT 5.1: The DOT 5.1 is a glycol-based fluid generally used in delivery trucks and heavy-duty trucks. This fluid is also employed in performance vehicles. It has the same dry and wet boiling point as that of the DOT 5 fluid.


Warning: Brake fluids of different types should not be mixed. They can react badly with each other and degrade the brake system.


Interchanging brake, transmission and steering fluids with one another can damage the internal parts of the systems. Keep in mind that these fluids possess different characteristics and are designed to be used in their specific systems.




  • The brake fluid is incompressible in its fluid state but becomes compressible in its gaseous form. When brake parts such as brake pads or rotors wear out, they can induce a significant amount of heat to the fluid. This can convert the fluid from liquid to gas, resulting in a complete loss of braking power.
  • The brake fluid, over time, can acquire debris from brake parts.
  • The brake parts can develop cracks, causing the brake fluid to leak out of them. This can in turn bring down the level of fluid.




Take a look at the level of brake fluid whenever you are inspecting your braking system. Generally, the recommended interval to replace the brake fluid is every 30,000 miles or every 2 years. However, for your vehicle's specific interval, please refer to your owner's manual, as the period for replacement can vary with the type of brake fluid.


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