A brake control device has a selective release accelerator which responds only to a normal release subsequent to an increase of brake line pressure initiated by the operator's brake valve. A first piston has one side subjected to air line pressure and the other side subjected to pressure in an auxiliary reservoir. The first piston controls an inlet valve to connect the emergency reservoir to the brake line. A second piston has one side subjected to brake line pressure and its other side to pressure in the auxiliary reservoir and monitors the first piston. The side of the first piston subjected to auxiliary reservoir pressure is connected through a throttled connection to the auxiliary reservoir when the brake line pressure is smaller than or equal to the auxiliary reservoir pressure. This throttled connection is interrupted when the brake line pressure reaches a predetermined value higher than the auxiliary reservoir pressure.
