Pedal pressure is amplified by the vacuum booster. The engine is an air pump with suction on the intake side. The low pressure in the intake manifold is applied to a rubber diaphragm on the vacuum side of the chamber (red speckled area) which then pulls the piston rod in when the brakes are applied to supply a power assist. The piston rod is connected to the brake pedal. A good check on booster operation is to apply light pressure to the pedal when the engine is cranking, the pedal should drop a little when the engine fires up.