A power steering system for a vehicle, comprises a steering shaft connected to
a steering mechanism. A hydraulic power cylinder is provided for assisting a steering
effort of the steering mechanism. A reversible pump is provided having first and
second discharge ports which are respectively connected through first and second
hydraulic lines with two hydraulic pressure chambers of the hydraulic power cylinder
so that the two hydraulic pressure chambers are respectively provided with hydraulic
pressures. A driving device is provided for driving the reversible pump to rightly
and reversely rotate. A differential pressure regulating valve is disposed in the
hydraulic circuit to maintain pressure at least in the first and second hydraulic
lines at a level of atmospheric pressure or higher in an inoperative condition
of the reversible pump.
