Patent ID: 7370571

Claim:
A negative pressure type booster device comprising: a booster shell movably supporting a partition member partitioning the interior of the booster shell into a variable pressure chamber and a constant pressure chamber; a valve piston secured to the partition member at a base portion thereof and having a negative pressure valve seat formed thereon for selective communication of the variable pressure chamber with the constant pressure chamber; an output rod connected to the valve piston; a reaction member for transmitting an output of the partition member depending on the pressure difference between the variable and constant pressure chambers, from the valve piston to the output rod; a plunger operable in connection with the reaction member and having an atmosphere valve seat formed thereon; an input rod connected to the plunger for axially moving the plunger when the input rod is axially moved by a brake pedal; a valve member having a negative pressure valve and an atmosphere valve formed thereon, the negative pressure valve being contactable with the negative pressure valve seat of the valve piston for making the variable pressure chamber communicate selectively with the constant pressure chamber, the atmosphere valve being contactable with the atmosphere valve seat of the plunger for making the variable pressure chamber communicate selectively with the atmosphere; a silencer disposed in a passage for leading the atmospheric air to the atmosphere valve; a secondary passage formed between an internal surface of a cylindrical portion of the valve piston and an external surface of the silencer to communicate directly with the atmosphere; and communication means having its path area which is controlled to vary in dependence on a moving amount of the input rod when the same is advanced relative to the valve piston beyond a predetermined distance, so that the atmospheric air from the secondary passage is led to the variable pressure chamber through the communication means.