Pneumatic servo booster

In a pneumatic servo booster of the type including a housing, a power piston unit partitioning the interior of said housing into front and rear chambers and having therein a generally longitudinally extending first passage having a first end communicated with said front chamber and a second end communicated with the interior of said power piston unit, and a generally radially extending second passage having a first end communicated with said rear chamber and a second end communicated with said interior of said power piston unit, a valve mechanism provided in said interior of the power piston unit and being associated with an input rod to control the communication between said first and second passages and between said second passage and another passage which is communicated with a source of a first reference pressure, said front chamber being communicated with a source of a second reference pressure which is lower than said first reference pressure, and an output rod connected to said power piston unit, the improvement wherein said power piston unit comprises:

• a power piston main body;
• a cover member secured to said power piston main body and covering said first end of said first passage;
• and
• a plurality of diversion channel means, provided between adjacent surfaces of said cover member and said power piston main body and extending radially outwardly from the periphery of said first end of said first passage and through said cover member, for communicating said first passage with said front chamber and for reducing noise caused by fluid passing from said first passage into said front chamber.