A servo is a system that works with a vacuum. In gasoline-powered cars, the air pump is connected to a pipe from the engine intake manifold to create a vacuum. In diesel cars, the vacuum pump pumps air into the vacuum. A vacuum pump is usually attached to the dynamo.
When you open the servo unit, you will find a large sheet of rubber called a diaphragm (about the size of a dinner plate). The difference in external pressure (global pressure) and the difference in vacuum pressure inside the intake manifold causes the pressure on the diaphragm to multiply according to the principle of Pascal’s law. The servo can increase the force applied by the brake pedal and push the master hive with great force.