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Brake fluid is a chemically-inert hydraulic fluid used to transmit force and motion. It also lubricates the piston in the master cylinder, wheel cylinders, and calipers. When applying brakes, this hydraulic brake fluid transfers force into pressure going onto the front and rear brake which will cause the vehicle to stop its movement.


  • The Department of Transportation classified the brake fluid into DOT 3, DOT 4, and DOT 5.
  • DOT 3 brake fluids are hygroscopic that absorb moisture mostly used in automobiles.
  • DOT 4 brake fluids are also hygroscopic and used for disc-brake systems.
  • DOT 5 brake fluid is silicone-based and can take even higher temperatures but is incompatible with DOT 3 or 4 fluid and must not be mixed with either of them.


  • Brake fluid must be stored in air-tight containers to protect it from moisture.
  • Brake fluid with different DOT ratings should not be mixed because it may cause corrosion