Hydraulic brakes play a critical role in the safe operation of any passenger vehicle.
Until the 1930s, most brake systems relied solely on mechanical force from the driver's foot applied to the brake pedal. Rods or cables directed this force to press brake shoes against a drum, which slowed the wheels. Hydraulic braking systems, in contrast, employ fluid under pressure to apply greater and more uniform braking forces. Pushing the brake pedal activates pistons contained in a master cylinder that transforms the applied force on the pedal into pressure throughout the hydraulic system, causing pistons in the wheel cylinders to activate either drum or disc brakes. Because the area of the hydraulic cylinders in the wheel is larger than the area of the master cylinder, the applied force is amplified.