What Is a Brake Fluid?
Brake fluid is a hydraulic fluid that transmits force from the brake master cylinder (operated by the brake pedal) through the brake lines to the wheel calipers and cylinders. Most vehicles use DOT 4 brake fluid, which must have a high boiling point to withstand the heat generated by braking without vaporising. Brake fluid is hygroscopic — it actively absorbs moisture from the atmosphere through the brake system over time.
What Does the Brake Fluid Do?
Brake fluid provides the incompressible hydraulic medium through which pedal force is multiplied and transmitted to the braking components. The boiling point of the fluid is critical — if fluid boils under heavy braking, vapour (which is compressible) replaces liquid in the brake lines, causing the pedal to go soft or to the floor (brake fade). Moisture absorption progressively reduces the boiling point of the fluid over its service life.
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Get My Free Fault Scan →Symptoms of a Failing Brake Fluid
Degraded or moisture-contaminated brake fluid causes a soft or spongy pedal, particularly under sustained heavy braking. In severe cases the pedal may travel to the floor. These symptoms may only appear during hard braking, masking the issue during normal driving. A low brake fluid level in the reservoir indicates either worn pads (normal) or a leak somewhere in the system (serious).
When Should the Brake Fluid Be Replaced?
Brake fluid should be changed every two years regardless of mileage — this is a safety-critical service item that is often overlooked. The cost is minimal — typically £30 to £60 for a full brake fluid flush at a garage. Testing strips can measure the moisture content of the fluid to determine whether a change is needed.
If you are experiencing symptoms related to your brake fluid and are unsure whether it needs replacing, a free MotorLoom fault scan can help identify whether the component is likely at fault - giving you the most likely cause and estimated repair cost before you visit a garage.
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Related Parts
Other components that work alongside or are related to the brake fluid: