Negative pressure boosting device

A negative pressure boosting device comprising:

• a shell having a space therein;
• a valve body which is disposed slidably relative to an inside of the space defined by the shell and air-tightly and slidably penetrates said shell;
• a power piston, connected to said valve body, for dividing said space into a constant pressure chamber into which negative pressure is introduced and a variable pressure chamber into which atmosphere is introduced during braking operation;
• a valve plunger which is slidably disposed in said valve body;
• an input shaft which connected to said valve plunger and is slidably disposed in said valve body;
• end
• a valve mechanism comprising a vacuum valve, which is disposed in said valve body and is controlled by a movement of said valve plunger to isolate or allow communication between said constant pressure chamber and said variable pressure chamber, and an atmospheric valve which is disposed in said valve body and is controlled by the movement of said valve plunger to isolate or allow the communication between said variable pressure chamber and the atmosphere, said vacuum valve including a valve element for receiving working pressure corresponding to input exerted on the input shaft to move in a working direction and a movable valve seat movable relative to the valve body, on which the valve element is seated;
• and
• servo ratio changing means for changing a servo ratio during the breaking operation after the input is initially exerted on said input shaft and before the input reaches an end of an intermediate load range, said servo ratio changing means being formed of said movable valve seat and a control spring for controlling a start of movement of the movable valve seat relative to the valve body.