Hydraulic power steering system

A hydraulic power steering system comprising:

• a hydraulic cylinder in which an interior is partitioned into a first hydraulic chamber and a second hydraulic chamber by a piston provided on a steered shaft;
• and
• an electric pump that supplies a hydraulic fluid to the hydraulic cylinder, and the hydraulic power steering system assisting a steering operation for steering a vehicle by moving the steered shaft together with the piston using a force produced due to a difference between a hydraulic pressure in the first hydraulic chamber and a hydraulic pressure in the second hydraulic chamber;
• a selector valve that is selectively set in an operation mode in which the hydraulic fluid is supplied to the first hydraulic chamber while the hydraulic fluid is drained from the second hydraulic chamber or in an operation mode in which the hydraulic fluid is supplied to the second hydraulic chamber while the hydraulic fluid is drained from the first hydraulic chamber;
• a detector that detects an operation state of the vehicle;
• and
• a controller that sets, based on the operation state detected by the detector, a fluid supply-drain mode in which the hydraulic fluid is supplied to and drained from the hydraulic cylinder, and controls the operation mode of the selector valve and an amount of hydraulic fluid that is supplied from the electric pump based on the set fluid supply-drain mode,
• wherein the controller controls, when a steering wheel is turned back toward a neutral steering position, the operation mode of the selector valve such that a drain of the hydraulic fluid from the hydraulic chamber of which a capacity decreases, from among the first and second hydraulic chambers, is suppressed.