An anti-lock brake system for a wheel wherein a fluid pressure is applied to a wheel cylinder, through a solenoid-operated valve device and a flow control device. Upon detection of locking of the wheel, the valve device is brought into an anti-lock state in which the valve device is alternately placed in its pressure-increase and pressure-decrease positions. The flow control device is normally placed in a first mode for a sufficient flow of the fluid into the wheel cylinder, and is operated in a second mode with the valve device placed in the anti-lock state. In the second mode, the flow control device decreases the rate of fluid flow to the wheel cylinder, with an increase in a difference between the power source pressure and the wheel cylinder pressure. An assisting device is provided for enabling the flow control device to adequaely effect the first mode of operation for at least a period between commencement of brake application and commencement of anti-lock operation of the valve device. The assisting device further assists the flow control device to adequately effect the second mode of operation at least while the valve device is in the pressure-increase position during the anti-lock operation of the valve decive.