What is a Brake Fluid?
The hydraulic liquid that transmits your foot on the pedal to the brakes at each wheel.
Simplified animation — not to scale.
In plain English
Brake fluid is a specialised hydraulic oil (typically DOT 4) that fills the brake lines between the master cylinder and the calipers. It's nearly incompressible, so pushing the pedal at one end instantly squeezes the pads at the other.
"Like the syrup in a hot drink — once it's soaked up enough water, it doesn't behave the same way under heat."
How it works
Pressing the pedal forces fluid out of the master cylinder, through steel pipes and rubber hoses, and into the pistons at each brake. Over time the fluid absorbs moisture from the air, which lowers its boiling point — when it boils, the gas compresses and your pedal goes soft.
Signs of trouble
- ⚠Spongy or long brake pedal
- ⚠Brake warning light on the dashboard
- ⚠Low fluid in the reservoir under the bonnet
- ⚠Brakes feel fine cold but fade after repeated use
- ⚠Fluid more than 2 years old or moisture content above 3%
£60 – £110 fitted
Always get a written quote. Prices vary by car, region, and parts brand.