Brake fluid is a vital part of your car’s braking system that allows the pressure from your foot on the pedal to reach the brakes at each wheel. If it is not working correctly, your braking performance will be reduced, even if the pads and discs appear to be in good condition.
Many drivers only think about brake fluid when something feels different. The brake pedal may feel softer than usual, or the car may not slow down as smoothly as it once did. These changes are easy to dismiss, but they often point to old, low, or contaminated brake fluid. Brake fluid transfers pressure through sealed brake lines and must remain clean to work effectively. Over time, it absorbs moisture and degrades, affecting braking performance and stopping distances.
This guide explains what brake fluid does, why it is important for safety, and how to recognise when it needs professional attention.
Brake fluid is the hydraulic liquid that transfers pressure from your brake pedal to the braking components at each wheel. When you press the pedal, the pressure moves through sealed brake lines to the callipers, helping the pads clamp the discs so the car slows in a controlled manner.
Most cars use glycol-based fluids such as DOT 3, DOT 4, or DOT 5.1, and the correct specification is set by the vehicle manufacturer. The Institute of the Motor Industry explains that glycol-based brake fluid is hygroscopic, meaning it absorbs moisture from the air [1]. Therefore, measuring water content is an integral part of routine maintenance. The same source notes that manufacturers commonly quote a 24-month lifespan for brake fluid in everyday use, and that since 2018, contamination has been part of the MOT inspection, so fluid condition matters as well as fluid level.
Moisture in brake fluid is problematic because it accelerates corrosion within the braking system. Under heavy braking, water in the fluid can vaporise, creating gas pockets that reduce hydraulic effectiveness.
Brake fluid is what allows the braking system to generate hydraulic pressure when you press the pedal. That pressure applies the brakes to each wheel. If the brake fluid is low or degraded, braking may feel less consistent, and the car may take longer to slow.
Brake fluid condition also matters because moisture and contaminants can degrade system performance over time. Moisture can contribute to corrosion within brake components and reduce braking effectiveness with repeated use, particularly when the system is hot.
Brake performance and the condition of the braking system are part of UK roadworthiness checks. The DVSA MOT inspection manual sets out the inspection rules and processes for class 3, 4, 5, and 7 vehicle tests. It also includes a dedicated section on brakes, including brake fluid rules and inspection [2].
If braking feels different or a warning light is on, a check during regular Service & Repairs allows technicians to assess the condition of the brake fluid alongside other braking components.
Low brake fluid reduces the hydraulic pressure available in the system. This often results in a softer brake pedal and slower braking response. In some cases, braking can feel inconsistent, particularly during repeated use.
Old brake fluid with higher moisture content is more likely to overheat during heavy braking. When this happens, braking effectiveness is reduced, and pedal feel may change. This is one reason fluid condition is checked during routine inspections.
Brake-related issues are often identified during an MOT, where braking efficiency and warning lights are assessed against DVSA standards. An MOT service from our team can highlight concerns early, before they develop into safety risks.
Brake fluid condition usually changes gradually, so issues are not always obvious straight away. Many drivers notice changes in how the brakes feel before any warning light appears.
Common signs that brake fluid may need attention include:
These signs do not always indicate that the fluid alone is at fault, but they do suggest that the braking system should be checked. During routine inspections, technicians can assess both fluid condition and braking components across different makes and models.
Brake fluid maintenance usually means replacing the old fluid, not simply topping it up. A top-up can bring the level back to normal, but it does not remove moisture already absorbed by the fluid, and it can delay detection of issues such as wear or a leak.
The RAC explains that brake fluid absorbs moisture from the air over time, which reduces its effectiveness during braking and its ability to protect components from corrosion [3]. The same RAC guidance recommends changing brake fluid every two years or every 24,000 miles, whichever comes first, while noting that the correct interval can vary by make, model, fluid quality, and driving style.
Brake fluid replacement should be carried out by a professional garage or mechanic. The RAC describes it as a delicate process, and the correct DOT fluid and method should be taken from the vehicle owner’s manual. A proper replacement also removes air from the system, helping restore consistent braking pressure.
Brake fluid helps your brakes respond properly when you need to slow down or stop. If the brake pedal feels different, a warning light comes on, or it has been a while since the fluid was last changed, it is worth getting the system checked by a professional.
At Sunrise Service & MOT Centre, we inspect the braking system, confirm the correct manufacturer-specified brake fluid, and check pedal feel once work is complete. We keep our advice clear and practical, so you know what needs to be done and why. You can read feedback from local drivers on our Testimonials, and you can also see the workshop and the kind of work we do in our Gallery.
Call us on 01258 459 798 or use our contact form to book a brake check at a time that works for you.
[1] The Institute of the Motor Industry, “glycol-based brake fluid is hygroscopic”: https://tide.theimi.org.uk/industry-latest/motorpro/boiling-point-brake-fluid
[2] GOV.UK, “The DVSA MOT inspection manual”: https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles
[3] The RAC, “brake fluid absorbs moisture from the air over time”: https://www.rac.co.uk/drive/advice/car-maintenance/brake-fluid-change-how-to-do-it-yourself/