What is a Brake Fluid?
The hydraulic fluid that transmits lever and pedal pressure through to the calipers.
Simplified animation — not to scale.
In plain English
Brake fluid fills the sealed system between master cylinder and calipers, carrying pressure through the lines. It's hygroscopic — it absorbs moisture from the air over time, which lowers its boiling point.
“The messenger carrying your hand's squeeze all the way to the caliper — but one that degrades and needs replacing periodically.”
How it works
Because fluid doesn't compress, pressure applied at the lever transfers through the lines almost instantly. Over time, absorbed moisture lowers the fluid's boiling point, which can cause a spongy or fading brake feel under hard, repeated braking — most manufacturers recommend a full fluid change every one to two years regardless of mileage.
Signs of trouble
- ⚠Discoloured, dark or cloudy fluid in the reservoir
- ⚠Spongy lever feel that wasn't there before
- ⚠Brakes fading noticeably during hard, repeated use
- ⚠Fluid level dropping without a visible leak
£25–£60
Always get a written quote. Prices vary by bike, region, and parts brand.