Skip to main content

Beginner's Guide: What Is Brake Fluid (and What Do the DOT Numbers Mean)?

Brake fluid types explained

What is brake fluid?

Brake fluid is a hydraulic liquid that is the lifeblood of your vehicle's braking system, and without it you simply won't stop in time.

It transfers the force you apply through the brake pedal through the various brake lines to the brake calipers and (if fitted) brake 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 are the different types of brake fluid?

There are several types of brake 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.

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).

Glycol-based brake fluid has two disadvantages: it is hygroscopic (attracts water) and needs to be changed every couple of years because braking efficiency will be reduced when it absorbs water.

Brake fluid also corrodes 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.