Vacuum actuated type booster device

In a vacuum actuated booster device having a booster shell, a booster piston reciprocatably received by said booster shell, a diaphragm connected at its inner peripheral portion to the rear side of a radial disc portion of said booster piston and at its outer peripheral portion to the peripheral wall of said booster shell, a first working chamber and a second working chamber defined in said booster shell by said booster piston and said diaphragm, a vacuum source with which said first and second working chambers are normally communicated, an input rod slideably mounted in said booster piston, and valve means connected to atmosphere for bringing said second working chamber into communication with ambient air by the forward movement of said input rod so that said booster piston is driven forwardly by the differential pressure between said first and second working chambers,

• an improvement wherein said booster piston comprises a central boss portion, an outer peripheral rim portion and a disc portion integrally interconnecting said boss portion and rim portion, said boss portion, rim portion and disc portion in combination forming a piston body, said disc portion being provided on its front surface with a plurality of radial protrusions and a plurality of windows between adjacent, protrusions, said windows being covered by porous thin-walled members which in turn are contacted by said diaphragm so that said windows are closed by said diaphragm.