Master Cylinder
A hydraulic device that converts pressure from the brake pedal into hydraulic force, sending brake fluid to the calipers or wheel cylinders.
The master cylinder is the heart of a vehicle's hydraulic braking system. When you press down on the brake pedal, it pushes a piston inside the master cylinder, compressing brake fluid and forcing it through steel lines to the calipers (on disc brakes) or wheel cylinders (on drum brakes). This hydraulic pressure is what ultimately clamps the brake pads or shoes to slow the vehicle.
Modern vehicles use a tandem master cylinder, which features two separate internal chambers and circuits. This design is a critical safety feature: if a leak develops in one brake line (for example, the front brakes), the other circuit (the rear brakes) will continue to operate, ensuring the vehicle can still be brought to a halt.
A failing master cylinder can lead to a dangerous loss of braking power. The most common symptom is a spongy or soft brake pedal that slowly sinks to the floor when constant pressure is applied. This is often caused by worn internal rubber seals allowing brake fluid to bypass the piston. External fluid leaks at the connection between the master cylinder and the brake booster are also common.
Replacing a master cylinder requires bleeding the entire brake system to ensure no air bubbles are trapped in the hydraulic lines, as air compresses and reduces braking efficiency.