You didn't say what year but if its an older truck with mechanical linkage going to the steer cylinder, it sounds like the spool valve is stuck in the cylinder. Very old truck may have the power steering pump mounted on the back of the generator. Make sure the generator is driving the pump. These couplings were known to wear out. If its a newer version with only hoses going to the cylinder check the pressure at the steering unit and see whats going in, then check to see whats coming out to the cylinder.