The centering valve is only in a power steering box, right? I think he said it has a manual box. It sounds to me like your steering box isn't set up right. I would check the idler arm as already suggested, but I would also disconnect the drag link from the pitman arm and make sure the box is set up right. Count the number of steering wheel turns from one extreme to the other. Turn the wheel back exactly half way. This will be the tightest position of the steering box (least clearance). The adjustment for this clearance is on top of the box, there is a jamb nut and adjusting screw. Adjust it until the clearance is just about zero. This should also be the position where you center the steering wheel, too. You may have to adjust your tie rod sleeves to straighten up the front wheels after you re-connect the drag link. I would also make sure you don't somehow have a lot of negative caster. If everything is good and you still have an issue, try changing to another pair of front tires just to rule them out.