Understanding the Difference Between DOT 3, DOT 4, DOT 5 & DOT 5.1 Brake Fluid

An effective braking system is a crucial part of any vehicle, especially in high-performance driving and towing. High speeds and heavy vehicles produce excessive heat when slowing down, and your brake fluid must be able to withstand those high temperatures while transferring force from the brake pedal to the calipers or wheel cylinders.

Brake fluid must meet specific safety standards set by organizations like SAE and DOT.

DOT 3, DOT 4, and DOT 5.1 brake fluids are all glycol ether-based. Glycol-based fluids are hygroscopic, meaning they gradually absorb moisture from the atmosphere and disperse it evenly throughout the system. As the water content of the fluid increases, its boiling point decreases. The additional moisture in the fluid will also start to corrode the system’s metal components.

DOT 5 brake fluid is silicone-based. Silicone-based fluids are hydrophobic, meaning they will not absorb
moisture. Any moisture introduced into the system collects in pockets that can freeze or boil off. Silicone is also more compressible than glycol, which can lead to a “spongy” feeling brake pedal.

DOT 5 was developed for longevity in braking systems where moisture is a factor.

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Because each brake fluid interacts with water differently, glycol- and silicone-based fluids are NOT compatible with each other, and the two fluid types should never be mixed.

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The boiling point is the most critical factor; brake fluid must withstand very high temperatures without vaporizing in the lines. If the fluid vaporizes, it will cause brake fade. The dry boiling point is the boiling temperature of fresh, new brake fluid from an unopened container. The wet boiling point is the temperature at which brake fluid will begin to boil after it has absorbed 3.7% water by volume. After approximately two years of use, brake fluid will reach this water volume level.

Brake fluid should be replaced during any brake service or around the two-year mark to maintain boiling
point and proper compressibility, and combat corrosion.

Brake Fluid Boiling Points