What is brake fluid and what does brake fluid do?
Brake fluid - sometimes known as hydraulic fluid - is crucial for moving parts of your braking system.
This fluid works under very high pressure and, assuming it is not contaminated, will not corrode internal components.
Without brake fluid, you would quite simply be unable to brake.
Brake fluid cannot be compressed. When you press your brake pedal a piston inside the brake calliper is compressed, which forces the brake fluid into motion.
The pressure from the brake fluid squeezes the brake rotors down onto the brake pads, which in turn make contact with the wheels causing your vehicle to slow and then stop.
Check how your brakes are performing with our guide here.
Note: You should always check your vehicle handbook for any fault remedy before carrying out any work on your vehicle. If you are unsure of what is required to fix any problems, please consult a professional.
How often do you need to replace brake fluid?
Most manufacturers recommend that you replace brake fluid every 24 months or around 24,000 miles, whichever comes first.
Over time, brake fluid absorbs moisture from the air. This lowers its boiling point and reduces braking performance, especially under heavy use or emergency braking.
Regularly flushing brake fluid helps keep your braking system responsive and reliable.
Brake fluid service
You can change brake fluid yourself, but many drivers choose a garage service.
A professional brake fluid change in the UK typically costs between £30 and £60. Prices may vary depending on the vehicle and whether a full system flush is needed.
Compared to other servicing costs, replacing brake fluid is relatively affordable and plays a big role in vehicle safety.
Brake fluid fill level
If you have flushed the system or topped up brake fluid, always fill the reservoir to the marked ‘min’ and ‘max’ lines.
Do not overfill. Too much fluid can cause pressure issues as the brakes heat up.
If the fluid level drops suddenly, it may indicate a leak. In that case, avoid driving and seek professional advice.
What is the best brake fluid to use?
It's worth investing in a high quality brake fluid. There are three main types available today, called D.O.T. 3, D.O.T. 4 and D.O.T. 5.
3 and 4 are the most common, but 5 is now widely available too.
The main advantage to D.O.T . 5 is that it doesn't corrode paint or absorb moisture. Consequently, D.O.T. 5 can lead to pools of water building up in your braking system.
Contaminated brake fluid and why it’s a problem
Brake fluid absorbs moisture when exposed to air. This contamination lowers the boiling point and can cause spongy brakes.
Water in brake fluid can also lead to internal corrosion, damage ABS components and reduce braking efficiency.
This is why flushing brake fluid at the recommended intervals is so important.
How to avoid brake fluid contamination?
If you decide to top up your own brake fluid it's a good idea to only buy a small bottle to avoid brake fluid contamination.
Fill up your brake fluid cylinder then throw the remainder away - because as soon as air comes into contact with the fluid it will become contaminated. This is the case even if you put the cap back on very quickly.
How to change brake fluid yourself
If you’re confident working on your car, this is a basic overview of how to change brake fluid. Always follow your vehicle handbook instructions.
What you’ll need
- Correct brake fluid type
- Jack and axle stands
- Brake bleed kit or clear tubing
- Spanner set
- Protective gloves and cloths
Step by step guide
- Locate the brake fluid reservoir and remove the cap.
- Use a syringe or turkey baster to remove as much old fluid as possible.
- Refill the reservoir with fresh brake fluid up to the ‘max’ line.
- Starting with the brake furthest from the master cylinder, bleed each brake in turn.
- Open the bleed valve, press the brake pedal slowly, then close the valve.
- Repeat until clean fluid flows through and no air bubbles remain.
- Keep checking the reservoir and top up as needed to prevent air entering the system.
This process is how to change brake fluid safely, but it can take time. If you’re unsure at any point, stop and seek professional help.
You may also find our guide on brake pedal vibrations useful if your braking doesn’t feel right after servicing.
Does your car judder when you brake? Read here for reasons why.
When to get professional help
If your brake pedal feels spongy, your car pulls to one side, or braking performance feels reduced, do not delay.
In these cases, it’s best to have a professional inspect the system and replace brake fluid properly using specialist equipment.
Ready to keep your brakes in good shape?
Regular checks and timely servicing help keep you safe on the road.
If you’d rather leave it to the experts, book a brake fluid service with a trusted garage or explore our full car maintenance guides to stay one step ahead.
