A hydraulic braking pressure control apparatus provided with a modulator for controlling the supply of control hydraulic pressure to a wheel brake of the vehicle, wherein a partition is provided between a first cylinder portion and a second cylinder portion which are formed in a casing, a first piston adapted to divide the first cylinder portion into an input hydraulic chamber on the partition side and a control chamber on the opposite side is secured to one end of a piston rod extending through the partition slidably and oil-tightly, and a second piston adapted to define an output hydraulic chamber on the partition side within the second cylinder portion is mounted on the other end of the piston rod for axial relative movement. The partition is provided with a valve mechanism capable of disconnecting communication between the input and output hydraulic chambers. The second piston is biased in a direction toward the partition, and by relative movement of the second piston with respect to the piston rod, the interior of the output hydraulic chamber is prevented from reaching an excessive negative pressure level during an anti-lock operation. The apparatus can function as a proportioning reducing valve by forming the first piston with a smaller diameter than the second piston.