Question

The following diagram shows a simplified hydraulic braking system of a car.

1. Explain how the system works, starting from when the driver presses the foot pedal
2. Why would the system not function properly if air leak into the cylinder? 

Answer 1

1. When the driver steps on the foot pedal, the force is transmitted through the level system to the piston in the master cylinder. The pressure produced in the master cylinder is transmitted through the fluid to the wheel (or slave) cylinders where the pistons in turn push the brake shoes outwards. When the brake shoes are pushed out, the brake-lining presses on the rotating wheel causing the car to slow down or to stop.
2. Air is compressible and so the force exerted would be partially used to compress the air instead of pushing out the break shoes.