An apparatus for use in turning steerable vehicle wheels (14, 16) includes first and second pistons (42, 44) which cooperate with a housing (46) to define first and second chambers (38, 40). A rotatable screw member (84) extends from one of the chambers (38) into one of the pistons (84). A valve (22) is connected with the screw member (84) and is operable between an unactuated condition and either one of two actuated conditions directing fluid pressure to the chambers. A steering linkage (18) is connected with the pistons (42, 44) at a location between the pistons. Upon operation of the valve (22) from the unactuated condition to the actuated condition, the pistons (42, 44) move in the chambers (38, 40) and turn the steerable vehicle wheels (14, 16). As the pistons (42, 44) move, a ball nut (86) connected with one of the pistons (42) effects rotation of the screw member (84) to operate the valve (22) toward the unactuated condition.