Despite being one of the most important components of a car’s braking system, brake fluid is often neglected, and many drivers are unsure how to check their car’s fluid, let alone replace it. Some motorists might find brake-related maintenance a little intimidating, but with a basic knowledge of what brake fluid is, how it works and how to maintain it, you can make sure your car’s braking system is always operating at peak performance.

Here, we provide a complete, no-nonsense guide to brake fluid, including what it does and how often you should service it.

What is Brake Fluid?

Brake fluid is a type of hydraulic fluid, meaning that it’s responsible for moving component parts within a system. When the brake pedal is pressed, the compressive effect builds pressure in the brake system, forcing the fluid to press down on the brake rotors, squeezing the brake pads to bring the car to a stop.

Because of the high-stress, high-friction nature of braking, brake fluid must operate perfectly at high pressures and temperatures. This makes it vital that brake fluid is regularly checked and maintained, ensuring good braking performance in any condition.

how to change brake fluid

What Affects Brake Fluid Performance?

One of the biggest things which can inhibit the performance of a car’s brake fluid is moisture. As brake fluid ages it can absorb small amounts of moisture from its surroundings, including the air – this is called hygroscopic. If brake fluid becomes too full of moisture, problems can arise when the fluid becomes hot under braking, with the moisture turning to vapour inside the brake lines. This leads to a soft, spongy brake pedal feel, and will stop your car braking as well as it should.

You should change the brake fluid at least once every two years to keep your brakes working properly. You should also take care when you’re checking or changing the brake fluid, to ensure the fluid doesn’t come into contact with excessive air and moisture.

What do Brake Fluid DOT Ratings Mean?

All forms of brake fluid are given a DOT rating. DOT simply stands for Department of Transport, which sets the safety regulations for the acceptable performance of different brake fluids. The DOT ratings given to brake fluids are based on the liquid’s dry and wet boiling points. As a rule of thumb, the higher the boiling point, the longer the lifespan of the brake fluid.

The table below shows the characteristics of DOT brake fluids:



Dry Boiling Point

Wet Boiling Point

DOT 2190°C140°C
DOT 3205°C140°C
DOT 4230°C155°C
DOT 5260°C180°C
DOT 5.1270°C180°C



While it’s better to use brake fluid with a higher DOT rating, you should always refer to your car’s manual, as some brake fluids are incompatible with different brake lines and systems. Your manual will let you know which type of brake fluid to use.

Can I Mix Different Brake Fluids?

One of the requirements of the DOT system is cross-compatibility, so brake fluid manufacturers must ensure their fluids won’t inhibit braking performance if they’re mixed with other products in a car’s braking system. This means that mixing different types of brake fluid won’t have any harmful effects. However, it’s always best to use the same type of brake fluid to guarantee optimum braking performance.

how to change brake fluid

How Often Should I Change My Brake Fluid?

Mechanics would recommend servicing your brake fluid every one to two years, to keep your brakes in good working order. Over time, the brake fluid will naturally absorb some moisture, so having them checked and serviced regularly will give you peace of mind that you won’t experience vapour in the brake lines. Every vehicle has different maintenance needs, however, so you may need to service the brake fluid more or less frequently depending on your annual mileage. Refer to your vehicle’s manual for an advised service schedule.

How Can I Top Up My Car’s Brake Fluid?

Here’s a step-by-step guide to topping up your car’s brake fluid reservoir:

1. Locate the brake fluid reservoir – This is mounted on the brake booster against the bulkhead; it is normally an opaque white plastic container.

How to change brake fluid step 1

2. Check the brake fluid level – Like the coolant/antifreeze reservoir, the brake fluid bottle has markings to help you check the level. These are usually in the form of a gauge, with ‘max’ or ‘full’ at the top and ‘min’ or ‘low’ at the bottom.

How to change brake fluid step 2

3. Top up the brake fluid – Add brake fluid until the level reaches the ‘max’ or ‘full’ marker. Never overfill, as this will put the system under unnecessary pressure. If you think there could be a problem with the braking system, remember to monitor the brake fluid level at regular intervals, and refer to a mechanic if you’re in any doubt.

How to change brake fluid step 1

At Prestone, our DOT 4 Brake Fluid is formulated to excel in extreme temperatures, offering a dry boiling point of 260°C and a wet boiling point of 150°C. Guaranteed for use in all braking systems, it provides complete corrosion protection, and reduces fluid vaporisation. For more information, visit the Prestone homepage.