Brake booster

A hydraulic brake booster (12) for use in a brake system (10) having a first housing (100) with a first bore (102) separated from a second bore (104). The first bore (102) retains a power piston (118) and the second bore (104) retains a control valve arrangement (170). A push rod (30) of an input arrangement (49) is connected to the power piston (118) and linked to the control valve arrangement (170) by a lever arrangement (50). A second housing (200) encloses the control valve arrangement (170), lever arrangement (50) and a projection (117) that extends from the power piston (118) to define a cavity or relief chamber (202) having a permanent low pressure. The input arrangement (49) supplies the control valve arrangement (170) with an input force in response to a braking force applied to the push rod (30) to regulate the communication of pressurized supply fluid from the second bore (104). The hydraulic brake booster is characterized in that the regulated supply fluid acts on the first piston (118) to develop a corresponding operational fluid pressure which is communicated to a first set of wheel brakes (14,14') while the regulated supply fluid is directly communicated to a second set of wheel brakes (18,18') to effect a brake application.