[Bob B.]

That is a common Saginaw heavy truck power steering setup. I drove an 1989 TopKick with the same system. The 2 hoses going to the box are for the control valve, which functions similar to the center link/pitman arm control valve used on old GM vehicles with the ram type power steering. If the steering goes all the way to lock the same direction you are turning, the control valve is likely sticking. If it is going the opposite direction you may have the lines from the control valve to the hydraulic cylinder crossed. If the lines are correct try flushing the system and use some power steering conditioner as LynnB says. Do you have a manual for this truck? They are on Ebay all the time.