When braking, step on the brake pedal, the pedal force after the lever amplification role in the control valve putter. First, the control valve push rod return spring is compressed, the control valve rod and the air valve column move forward. When the control valve is moved forward to the control valve skin bowl and the vacuum one-way valve seat in contact with the position, the vacuum check valve is closed. At this point, the booster's true air chamber, the application of the gas compartment is separated. At this point, the end of the air valve column is in contact with the surface of the reaction plate. As the control valve push rod continues to move forward, the air valve opening will be opened. After the air is filtered through the open air valve and access to the application of the air chamber, access to the booster's application gas chamber (right chamber), servo force generation. Because the material (rubber) of the reaction plate has the physical property requirement of equal unit pressure everywhere on the surface of the force, the servo force increases with the increasing of the input force of the control valve rod. Because of the limitation of the servo force resources, when the maximum servo force is reached, that is, the vacuum degree of the application chamber is zero (i.e. a standard atmospheric pressure), the servo force will become a constant, no longer changes. At this time, the Power booster input force and output force will increase in equal amount, when the cancellation of braking, with the reduction of the input force, the control valve after the push rod, the vacuum valve opened, the booster's true air chamber, the application of the gas chamber, the servo force reduced, the piston body back. As the input force gradually decreases, the servo force will be reduced to a fixed proportion (servo force ratio) until the brake is completely relieved.