The Hydraulic Brake
Liquids are irtually incompressible, so they transmit pressure very effectively without loss. This has a number of applications. Maybe the most common is in automotive braking. A hydraulic braking system of the sort used in cars is shown below.
When the brake pedal is pushed, brake (hydraulic) fluid is forced along the pipes. The fluid forces the brake shoes into contact with the brake pads. A lot of friction is created and the wheels start to turn more slowly.
All four wheels on a car have the arrangement shown above for one wheel, and the hydraulic system has the important advantage of transmitting an equal braking force to all four wheels.
It is important to keep the braking system free of air. If air gets into the system, the pushing the brake pedal will merely cause the air to be compressed. The brake shoes are not forced into contact with the brake pads and no – or a much reduced - braking force is applied. To remove air from the system, the brakes must be bled.