Brakes are an incredibly important part of any car, but they don't work without brake fluid. By the end of this article, you will understand:

  • What brake fluid is
  • What role it plays in your braking system
  • The different types of brake fluid and which type your car needs
  • And how to check and top-up your brake fluid

 

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Glossary

Hydraulic Fluid - A type of liquid used to transfer power in hydraulic machinery. Brake fluid is a type of hydraulic fluid.

'Losing' brakes - The term used for when your brakes no longer slow your car down. This can mean the pedal goes all the way to the floor or your brakes feel spongy.

Glycol-Ether - A group of organic solvents used in industry to make many things, such as paint, cleaner and brake fluid.

Silicone-based - A synthetic rubber-plastic hybrid compound that forms the base for many things, such as sealant, lubricant and some types of brake fluid.

Hydrophobic - Simply meaning, 'afraid of water'. A hydrophobic substance will repel water rather than attracting or absorbing it.

Anti-Lock Braking System - A braking system fitted to many modern cars to prevent drivers from skidding under heavy braking. It reduces the chances of your wheels locking and you losing control.

DOT - Department of Transport. A brake fluid categorisation system originally designed in America but is now standard across much of Europe, including the UK. This helps drivers identify what type of fluid they need.

Dry Boiling Point - The minimum temperature needed to change brake fluid's state of matter from a liquid to a gas with no water present.

Wet Boiling Point - The minimum temperature needed to change brake fluid's state of matter from a liquid to a gas with 3.7% or more water present in the fluid. Brake fluid is described as wet when there is 3.7% or more water content in it.

 

What is Brake Fluid?

Brake fluid is the hydraulic fluid used in the braking systems of cars, HGVs and other vehicles. It is usually a hygroscopic fluid, which means that it absorbs water from the humidity in the air. 

Absorbing all this water may sound bad, but it actually helps your brakes perform better. Braking generates an extreme amount of heat and the fluid is designed to work under these conditions. This is because it has a much higher boiling point than water. Even when it is diluted by water, known as the 'wet boiling point', brake fluid has a boiling point of over 140°C.

If water pooled in the calliper rather than being dispersed throughout the entire braking fluid, you would 'lose' your brakes. This could have serious consequences.

 

a coloured diagram of a car braking system

Your car's braking system is quite complex but one thing is perfectly simple: it wouldn't work at all without the presence of brake fluid!

 

 

What Does Brake Fluid Do?

Brake fluid transfers force into pressure within the braking system. Simply put, your brakes won't work without this hydraulic fluid.

When you push the brake pedal, fluid is forced down to your brakes. This, in turn, forces your brake pads onto the brake discs. These squeeze the brakes against the wheel hubs, slowing down the spinning of the wheels. No brake fluid means no pressure which means your car won't slow down.

And, while your handbrake is there as an emergency resort, it's not strong enough to slow you down fast enough. 

 

What is Brake Fluid Made Of?

Brake fluid is either Glycol-ether (hygroscopic) or Silicone-based (hydrophobic). 

Glycol-ether is the standard base, while Silicone brings several performance advantages. The only issue with Silicone-based brake fluid is that it doesn't work well with Anti-Lock Braking Systems. As this is a common safety feature on modern, mass-produced cars, you must use the correct type of fluid for your car. 

You can find your designated brake fluid type in your vehicle handbook. It will be indicated by one of 4 'DOT' numbers.

 

What Brake Fluid Do I Need?

You must know what type your car needs and you should only use the recommended type as the different standards are not really compatible. While you could use a different type of Glycol-ether fluid in an absolute emergency, you can't use silicone-based brake fluid in a car not designed for it, and vice versa.

 

Dot 3 Brake Fluid

This has the lowest dry and wet boiling point and is designed for everyday use. It is mainly found in mass-produced cars with small to medium-sized engines.

 

Dot 4 Brake Fluid

This is the most common form of brake fluid. It is very similar to DOT 3 but includes additives that increase the minimum dry and wet boiling points. This increases the dry and wet boiling points. However, this also means that you need more frequent brake fluid changes. If DOT 4 is the recommended fluid, the manufacturer will suggest shorter intervals than other types of fluid. This is due to its increased rate of water absorption. 

DOT 4 is most suited to regular vehicles, especially those with larger sized engines. You can also find Super DOT 4 fluids which are specifically designed for racing or performance cars.

 

Dot 5 Brake Fluid

DOT 5 is the only silicone-based brake fluid and is NOT compatible with any car designed to run on DOT 3, 4 or 5.1. It is also extremely expensive due to its performance benefits. Unless your car is designed to use DOT 5, or you have a very specific reason to use it, you can ignore this type of fluid.

It is most suited for cars that are likely to have a lot of water present in the braking system. DOT 5 is the recommended fluid for many classic car drivers because it removes the concern of brake parts corroding while in storage. However, it can make your brakes feel spongy as it is more compresisble. As a result, it is not suitable for vehicles driven at high speeds or under racing conditions.

 

Dot 5.1 Brake Fluid

DOT 5.1 was created when it became clear that silicone-based brake fluids wouldn't become mainstream. It gives the same performance benefits but is a Glycol-ether based fluid. This makes it compatible with modern cars that use Anti-Lock Brakes. 

As a result, it is not 'better' than DOT 4 for any application, mainly because it is chemically similar to both DOT 3 and 4. It is designed for cars with larger engines or those used for sports driving in cold climates. This is because it has the joint highest dry and wet boiling point (with DOT 5).

 

How Do I Top Up Brake Fluid?

If you need to top yours up, it could indicate a leak or larger problem. You should check your brake fluid regularly, following these steps:

 

close up shot of brake fluid tank with light brown liquid in it

 

  1. Locate the main brake reservoir (as above)
  2. Unscrew the cap. This will let you look at the fluid level accurately. 
  3. If the fluid level isn't within half an inch of the cap, add the correct type of brake fluid until it is. The more fluid you have to add, the more serious the problem could be.
  4. You should also check your fluid's condition at the same time. It should be translucent and yellowish in colour. Anything else and you need to book a brake fluid change as soon as possible.

You should always work quickly when topping up your brake fluid. It will absorb the moisture in the air and become contaminated, so only buy and use as much as you need. You shouldn't use brake fluid that has been open for a long time either as this will also reduce the efficiency of your brakes. 

 

If you check your brake fluid and suspect that you need a top-up, you should book an appointment with a professional garage rather than attempting any of the work yourself. They will also be able to advise on any leaks that you might have and be able to fix these as well.

 

 

Recap

Brake fluid is a hydraulic fluid used in most modern vehicles. It transfers braking force into pressure to make your brakes work and slow your car down. It also absorbs moisture from the air to keep the boiling point as high as possible - else you would 'lose' your brakes.

You must check your brake fluid regularly and ensure you are using the right DOT type. This will keep you in control of your car and reduce your chances of an accident. If it is anything other than yellowish and translucent, or the level is more than half an inch below the reservoir cap, you should book a brake fluid change at a garage near you.