Final answer:
Low-pressure readings in a hydraulic power steering system can be caused by a worn hydraulic steering pump, a relief valve that is stuck closed, or a restricted steering valve; a bent pitman arm typically does not influence pressure.
Step-by-step explanation:
Low-pressure readings when resting a hydraulic power steering system can be caused by a variety of factors, but there are a few common culprits. Firstly, a worn hydraulic steering pump can result in inadequate pressure generation and thus low system pressure. Secondly, if a relief valve is stuck closed, it may lead to insufficient flow through the system, resulting in lower pressure readings. Thirdly, a restricted steering valve can prevent fluid from moving freely, which would also result in a loss of pressure. Lastly, while a bent or damaged pitman arm might affect steering performance, it is not typically related to the hydraulic system's pressure. Of the options listed, a), b), and c) are potential causes of low-pressure readings, whereas d) is not typically responsible for this issue.