A hydraulic chuck assembly for a tool machine includes first and second valve units disposed to control flow of pressurized fluids into a cylindrical chamber so as to move an annular piston to thereby move a taper socket along a spindle axis. Each valve unit includes a ball movable along a valve axis radial to the spindle axis such that the ball can be further forced to abut firmly against a valve seat during a high-speed rotation. Oil leakage can be prevented to ensure firm engagement of a workpiece with a collet head.