Master cylinder

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Master cylinder: a brake and clutch component that turns pedal force into hydraulic pressure. When you press the pedal, a piston moves inside the master cylinder, pushing fluid through lines to the slave cylinders at the wheels (for brakes) or on the clutch. The pressure and movement depend on the relative sizes of the master and slave pistons. Fluid travels through flexible hoses near each wheel and often hard lines elsewhere. A reservoir above the master keeps enough fluid and prevents air from entering. Most cars have two hydraulic circuits for safety: a diagonally split setup so one circuit powers front left and rear right, the other powers front right and rear left. If one circuit leaks or a line fails, the other circuit can still stop the vehicle, though braking will be reduced. With only one working circuit, braking distance increases. When checking brakes, uneven wear on pads or rotors can signal low pressure or a failing circuit.