DE2335529

A hydraulic brake booster is disclosed which includes a housing defining a pressure chamber therewithin in which a boost piston reciprocates and which is also provided with an inlet port connected to the fluid pressure source, an outlet port, and a return or exhuast port communicated to a fluid reservoir. A spool valve is slidably mounted in the housing and is shiftable from a first or brake released position in which the pressure chamber is vented to the reservoir and in which substantially all of the fluid flowing to the inlet is communicated directly to the outlet, to a second position in which fluid flow between the inlet and the outlet is severely restricted and the high pressure fluid at the inlet is communicated into the booster pressure chamber where it acts upon the boost piston to effect a brake application. A by-pass passage is provided between the inlet and outlet port to insure a minimum level of fluid communication between the inlet and outlet even when the spool valve is disposed in the second position. The by-pass passage includes a first flow-restricting orifice which communicates the inlet with a cavity, and a second flow-restricting orifice or diffuser which communicates the cavity to the outlet ports. The two orifices are so designed that the pressure drop across the first orifice is much greater than the pressure drop across the second orifice or diffuser, thereby providing a substantially noiseless valve which may be easily manufactured.