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Beginner’s guide: How brake fluid works and what are the DoT numbers?

Brake fluid types explained

Brake fluid is a hydraulic liquid that transfers the force you apply through the brake pedal through the various pipes to the calipers and (if fitted) drums, which then operate to slow your vehicle.

In such a closed system, the fluid resists compression, so every bit of force you exert with your foot is transferred to the stopping end of the set-up. 

What brake fluid do I need?

There are several types of fluid, grouped under two headings: those that are glycol-based and those based on silicone.

The glycol-based brake fluids include DOT3, DOT4, Super DOT4 and DOT5.1. DOT5 is a silicon-based brake fluid. Most cars use DOT4, but always check your handbook for guidance.

In general, the lower the number, the lower the fluid’s boiling point. In normal operation, brake fluid is a liquid and can’t be compressed, so the brake pads and shoes can be properly applied.

However, under hard driving, for example after a few laps around a race track or when descending a long, steep hill, the high temperatures created by the friction of the pad on the rotor can cause the brake fluid to boil and it starts turning into a gas.

This means it becomes compressible, which makes the brake pedal feel spongy, and reduces braking efficiency. This reverses once the brake fluid cools.

What brake fluid do I need?

This is why having the correct brake fluid for the driving conditions is vital.

Brake fluid wet boiling points

  • DOT3 140C (284F)
  • DOT4 155C (311F)
  • Super DOT4 195C (383F)
  • DOT 5.1 185C (365F)

Most types of brake fluid corrode painted surfaces, so be careful when topping up and changing the fluid.

DOT5 brake fluid is based on silicone and has a wet boiling point of 185C (365F). It doesn’t absorb water or harm painted surfaces but can’t be used in road cars because it doesn’t lubricate ABS pumps.

In addition, DOT5 compresses more than glycol brake fluids, giving a spongier brake pedal.

Never mix brake fluid types (even glycol-based ones) and always replace the lid on the bottle. We recommend buying fresh brake fluid whenever you top up or change it, to avoid possible water contamination.

Can brake fluid go bad?

Can brake fluid go bad?

Yes. Glycol-based brake fluid is hygroscopic, which means it attracts moisture. The longer brake fluid is left in a car’s braking system, the more water it will absorb. The symptoms of this will be a spongier feeling when the brake pedal is pressed.

It will become harder to apply the brakes properly and could lead to brake failure – and the water in the fluid could corrode the brake lines.

This is why it's important to change brake fluid every couple of years.