Abstract: A copy valve for an anti-lock brake system. The copy valve has a housing enclosing four chambers. A first and fourth chamber of the enclosed four chambers receives pressurized fluid directly from the first and second cylinders of a dual master brake cylinder. The third chamber receives the pressure of the controlled brake actuator while the second chamber is connectable to the uncontrolled brake actuator. In one embodiment, a single piston is enclosed in the housing and the first and second chambers are disposed at the opposite ends of the piston. The piston has an enlarged collar intermediate the two ends which is received in an annular recess provided in the housing. The third chamber is formed between a first face of the piston's enlarged collar and the annular recesses while the fourth chamber is formed between the opposite face of the piston's enlarged collar and the adjacent face of the housing. A throughbore is provided through the piston connecting the first and second chambers.