case 580b ck steering lost

I have a older case 580b ck forklift and the steering gradually was going out, so I changed the hand valve the the steering wheel mounts on and the PS pump-also took apart the steering control valve (which looked ok) But steering will not operate with the the wheels on the groung-but if i jack up up the axle and take off the weights=it will work fine.
Does anyone have any ideas on what to do nex ?

case 580b ck steering lost