A valve for reducing pressure in a hydraulic brake line upon skidding of a vehicle wheel. The valve is constructed with a bore having one end portion interconnected with the brake line, and a piston slidably disposed in the bore so as to form a chamber in communication with the brake line. The opposite end portions of the bore are in communication so that equal brake actuating hydraulic fluid pressure is exerted on each end of the piston. The valve contains a pressure differential operated member such as a piston or diaphragm which is operable to move the piston so as to seal the brake line and increase the volume of the chamber thereby reducing pressure in the brake line.