Understanding Brake Fluid 101

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 guide to brake fluid, including what it does and how often you should service it.

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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 all conditions.

brake fluid cap

What Affects Brake Fluid Performance?

One of the biggest things that 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:

DOT Dry Boiling Point Wet Boiling Point
DOT 2 190°C 140°C
DOT 3 205°C 140°C
DOT 4 230°C 155°C
DOT 5 260°C 180°C
DOT 5.1 270°C 180°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. Using the same brake fluid will ensure lasting durability and performance, so you can feel confident that your brakes are working at their best.

mechanic filling up brake fluid

How Often Should I Change My Brake Fluid?

To keep your brakes in good working order, mechanics would recommend servicing your brake fluid every one to two years. Over time, the brake fluid will 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.

Signs that You Should Change Your Brake Fluid

As well as pedal feel, there are a few other signs that tell you when it’s time to change your brake fluid, including:

  • Strange noises – does your car make a loud noise under braking? It could be time to change the brake fluid. Noise under braking could mean your brakes are down on fluid, so it’s important to get it checked before any damage occurs to the brake system.
  • A burning smell – when your brakes overheat, there’s a good chance you’ll smell it in the cabin. Hot brakes smell like burning, so this should set alarm bells ringing. Overheating fluid can lead to brake failure, so you must pull over if you suspect your brakes are running a little too hot.
  • ABS light – one of the simplest ways to tell when there’s a problem with your brakes is when the ABS light comes on. Most modern cars have an ABS system, and it can detect when there’s a problem with the brake fluid. As soon as you see the ABS light, get to a mechanic as soon as possible.

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. Park on a flat surface – before you pop the bonnet, make sure you’re parked somewhere level. A slope could give you an inaccurate reading when it comes to checking your brake fluid level
  2. Locate the brake fluid reservoir– This is mounted on the brake booster against the bulkhead; it is normally an opaque white plastic container.
  1. visual graphic 1 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.
  1. visual graphic 2 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.
  1. visual graphic 3 Replace the filler cap and go for a short drive – when you’re happy that the system is full, replace the cap, close the bonnet, and go for a quick drive. This circulates the brake fluid through the system and gives you the chance to check that the brakes are working as they should.

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.

Can I Replace Old Brake Fluid?

If your brake fluid hasn’t been changed for several years, you may need to replace it to ensure optimal braking performance. Follow the steps below to replace old brake fluid quickly and safely:

  1. Assemble the right kit – for this job, you’ll need a container, a turkey baster, and a lint-free cloth. We’d also recommend rubber gloves.
  2. Remove as much old fluid from the reservoir as you can – with the turkey baster, remove as much fluid from the brake fluid reservoir as possible. Make sure you have a container to hand to pour the old fluid into, and don’t let it touch any painted surfaces.
  3. Refill the system with fresh brake fluid – fill the system to the ‘max’ indicator with good quality brake fluid. Remember, the less exposure to the air that brake fluid has, the longer its life, so work as quickly as you can.

mechanic checking brake fluid

  1. Bleed the brakes to remove the remaining old fluid – when the system is full, it’s time to bleed the brakes to force the old fluid out of the system. If you’re not sure how to do this, follow the quick steps below:
  • Locate the bleeder screw behind each brake; you may need to jack up your car to do this.
  • When you’ve located the bleeder screw, you need the right tool to loosen it. A screwdriver or wrench may work, or you can buy a specialist bleeder wrench to make the task easier. Don’t loosen it yet, though.
  • Before you loosen the bleeder screw, you need to do two things: first, get a friend to sit in the driving seat and ask them to press the brake pedal when you’re ready. Second, have a container to hand to catch the brake fluid that squirts from each bleeder screw.
  • When you’re ready, have your friend press the brake pedal and slowly loosen the bleeder screw. The old fluid should squirt out into the container. Before they release the pedal, retighten the screw so that air isn’t drawn into the brake system.
  • Repeat these steps until you’ve removed the old fluid (which will look dirty compared to the new). You’ll then need to do the same for the other three wheels.
  1. Recheck the brake fluid level and top up if necessary – when you’ve successfully bled all four brakes, recheck the brake fluid level on the reservoir; there’s a good chance you’ll need to top it up to the ‘max’ line with new fluid.

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.

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