Abstract: A hydraulic pressure control valve assembly for a hydraulic brake system for a wheeled vehicle, comprises a proportioning valve having a plunger which is axially movable in a direction against a spring in response to application of a hydraulic pressure to a fluid inlet port to control the magnitude of hydraulic pressure in a fluid outlet port, and a deceleration sensing valve responsive to a deceleration of the vehicle the degree of which exceeds a predetermined value. The deceleration sensing valve has a piston which supports an end of the spring and is sealingly disposed in a chamber to define a sealed expandable chamber which is communicable with the fluid inlet port to enable the expandable chamber to contain a fluid the pressure of which is changeable in accordance with the degree of the deceleration of the vehicle.