Patent ID: 7448308

Claim:
A vacuum 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 vacuum 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 vacuum valve and an atmosphere valve formed thereon, the vacuum valve being contactable with the vacuum 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 sound-absorbing member fitted at its outer surface in the internal surface of the valve piston and having an internal surface which is sufficiently larger in diameter than the input rod for defining a bypass passage which leads the atmospheric air to the atmosphere valve inside the valve piston without passing through the sound-absorbing member; and a sliding-type plate valve composed of at least two plate valve parts which are relatively slidable in radial directions and separable from each other in an axial direction, wherein the at least two plate valve parts are held in a face-to-face contact in a first operation state for blocking the communication of the bypass passage with the atmosphere valve inside the valve piston, but are separated axially from each other in a second operation state that the input rod is advanced beyond a predetermined distance relative to the valve piston, to establish a radial passage which makes the bypass passage communicate with the atmosphere valve inside the valve piston.