A Guide to Brake Fluid

General Brake Fluid Information

Brake fluids are one of the most important fluid in cars and bikes, so should be well looked after. They must have certain properties that make them suitable for their purpose.

1. Thin viscosity, even at lower temperatures (they are tested at -40oC).
2. Must be compatible with the seals in the braking system.
3. Reasonably non-compressable.
4. High boiling point.
5. Good at not absorbing water.
6. A good lubricant of moving parts.
7. Long-Lasting

To achieve these properties, modern brake fluids are made of glycol ethers and borate esters, mixed with various corrosion inhibitors. They are completely unlike any oil used in engines

Boiling Points

There are two boiling points to consider with brake fluids, the dry boiling point and wet boiling point. The dry boiling point is where the brake fluid contains no water and the wet boiling point is where the fluid has absorbed 3.7% of it's volume in water. That small amount of water cant affect the boiling point quite drastically, around 80oC in some fluids. When the fluid boils, it is vaporised and can then be compressed in the brake lines, reducing the braking force

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Types of Brake Fluid

DOT 3

This is the normal specification for many of the cars on the road today, with boiling points in the region of 200oC (dry) and 140oC (wet). This type of fluid should be changed in accordance with the manufacturers guideline

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DOT 4

This is also a common specification for road cars, but with higher boiling points than the DOT3 fluids (230-270oC dry, 160-170oC wet) where brakes are likely to generate more heat, for example, performance road cars. As with the DOT3 fluids, these ones are glycol based.

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DOT 5

These are not relevant to road use. They are silicon based, unlike the other brake fluids listed and are hydrophobic, so they do not absorb water and are therefore extremely long life fluids. These fluids are not suitable for the vast majority of the vehicles in use today, but are for certain military vehicle purposes, as well as a few other non-mainstream applications.

The boiling points for DOT5 fluids are approximately 260oC dry and 180oc wet. The 'wet' boiling point is slightly different to normal brake fluids as water cannot enter the fluid, it can only be a 'wet' fluid if there is water in the braking system

DOT 5.1

This is another glycol (unlike the DOT5 fluids) based fluid that is used in normal road and performance cars. Due to the formulation of the fluid, it is longer lasting than the DOT3 and DOT4 fluid, although the boiling points may not be much higher than the DOT4 fluids at around 270-280oC dry and 185oC wet. Do not confuse these fluids with the much rarer DOT5 ones

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SUPER DOT 4

These are the top performance brake fluids available. They are like a DOT4 in terms of the chemical components used and life span of the fluid, but they are formulated to give extremely high boiling points (generally in excess of 300oC dry and 190oC wet). These are the fluids that are used in the top levels of motorsport, but are still suitable for road use, so they are ideal for vehicles used on road and track

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