A power steering system and a method of charging the power steering system with hydraulic fluid are provided. Hydraulic fluid is selectively supplied to hydraulic chambers of a power cylinder via oil passages from a reversible pump, according to a steering torque. In addition, a bypass passage is provided between oil passages. A normally open switching valve is provided in the bypass passage. When the system is assembled, the hydraulic fluid is supplied from an oil reservoir via the switching valve or the reversible pump, and circulated in the hydraulic circuit. The air in the hydraulic circuit is discharged via the switching valve.