A valve assembly including a valve body having a first pressure chamber and a second pressure chamber, the pressure of the first pressure chamber being lower than that of the second pressure chamber. The pressure of the second pressure chamber is discharged to the first pressure chamber in response to the change of the pressure of the first pressure chamber. A blocking device is arranged between the first pressure chamber and the second pressure chamber. A valve stem is arranged in the valve body with one end movably contacting the blocking device. A diaphragm is connected to the valve stem and is arranged adjacent to or above the first pressure chamber. When the pressure of the first pressure chamber is changed, the diaphragm forces the valve stem to move by sensing the pressure change of the first pressure chamber.