In a vehicle braking system, a fluid pressure control device comprises an inertia-controlled valve for interrupting fluid communication between a master cylinder and rear wheel brake cylinders at a deceleration in excess of a predetermined value and a differential piston for gradually increasing the fluid pressure trapped within the wheel brake cylinders after the valve is closed. The control device is provided therein with a spring which is detachably engaged at opposite ends thereof with the small and large diameter portions of the piston in such a manner that the piston is urged toward an outlet chamber by engagement with the spring when moved toward an inlet chamber and also urged toward the inlet chamber by engagement with the spring when moved toward the outlet chamber.