Balanced servomotor valve

Claim 1

In a power brake apparatus having a servomotor with a control means for transferring an output force of a wall means from a first piston means to a concentric second piston means to produce an additional operational force to activate the wheel brakes in a vehicle in response to a corresponding operator input force, said control means comprising: a housing having a first bore and a second bore therein, said first piston means being located in said first bore to establish a first pressurizing chamber and a relief chamber, said second piston means being located in said second bore to establish a second pressurizing chamber, said second piston means having a first axial bore and a second axial bore separated by a fixed wall, said second piston means having a first radial bore and a second radial bore located on opposite sides of said fixed wall to permit communication between the first pressurizing chamber and the first and second axial bores, said second piston having a third radial bore for communicating said second axial bore to said relief chamber; first valve means located in said first axial bore of said second piston means for permitting fluid under pressure to only flow from the first pressurizing chamber into the second pressurizing chamber; second valve means located in said second axial bore of said second piston means, said second valve means having a cylindrical body with a first face and a second face positioned on opposite sides of said second radial bore to form a control chamber within the second axial bore, said cylindrical body having a passageway through which fluid in the relief chamber is communicated to the first face; stop means located in said second axial bore of the second piston means between the second radial bore and the third radial bore; first resilient means located in said second axial bore for urging said second face against said stop means to prevent communication between the control chamber and the relief chamber; and actuator means initially responding to an operator for developing said output force across the wall means of the servomotor which will move the first piston in the first pressurizing chamber and the second piston in the second pressurizing chamber to produce a first pressurizing fluid force, said first pressurized fluid force produced in the first pressurizing chamber being simultaneously transmitted into the first axial bore and the second axial bore of the second piston means for communication to the second pressurizing chamber and to said control chamber, said first pressurized fluid force in the control chamber and the fluid in the relief chamber acting across the first face and the second face of the second valve means to develop pressure differential forces which cancel each other, said first resilient means alone providing the closure force which prevents communication between the control chamber and the relief chamber, said actuator means further responding to the operator to overcome said first resilient means to allow fluid under pressure to be communicated into the relief chamber and permit said output force of the wall means of the servomotor to move the second piston means in the second pressurizing chamber to develop said additional output force.