So I have an '03 Navigator. The power steering is occasionally weak. It's not so hard as if it was not working at all but "sticky". I tried disconnecting the return line anf turning the engine over to flush the system with new fluid but nothing is moving. so I though perhaps I have the wrong line. But nope there is nothing in the reservoir either. So it appears as though the fluid is not moving. But the pump is not whining. The fluid is full. and it only occasionally has an issue. any ideas?