in a vehicular power steering system , the hydraulic fluid supply to a hydraulic power actuator is controlled in accordance with the vehicle speed and the angular velocity of the steering wheel . preferably , the fluid supply is reduced so that the steering effort becomes heavier as the vehicle speed increases while the fluid supply is increased when the angular velocity of the steering wheel is high . to control the fluid supply , there is provided , in a power steering system , a bypass valve means for draining a portion of the fluid flow to bypass the power actuator , and control means for producing a control signal to control the bypass valve means in accordance with the vehicle speed and the angular velocity of the steering wheel .