What is a Master Cylinder?
The pump behind your brake pedal that turns your foot push into stopping power.
Simplified animation — not to scale.
In plain English
The master cylinder sits under the bonnet, bolted to the brake servo. It's the heart of your hydraulic brake system — it converts the force from your foot into pressurised brake fluid that travels down to every wheel.
"Like a bicycle pump for your brakes — push the handle (pedal) and pressurised fluid (instead of air) does the work at the other end."
How it works
When you press the brake pedal, a piston inside the master cylinder pushes brake fluid out through steel pipes to each caliper. Because liquid can't be compressed, that pressure arrives almost instantly at all four wheels. Modern cars use a "dual-circuit" design, so if one half fails you'll still have brakes on the other two wheels.
Signs of trouble
- ⚠Brake pedal feels spongy or sinks slowly to the floor
- ⚠Brake fluid level dropping with no visible leak at the wheels
- ⚠Brake warning light on the dashboard
- ⚠Brakes feel uneven or weak even after bleeding
£200–£450 fitted
Always get a written quote. Prices vary by car, region, and parts brand.