<p>A brake system includes an operator actuated direct apply brake valve which channels fluid from a pump to the wheel brakes. An extremal anti-lock controller includes a transmission governor providing a fluid pressure equal to the average rear wheel speed and an accumulator which receives and stores fluid from the governor. Return of fluid from the accumulator to the governor during wheel deceleration is through a deceleration orifice which restricts the flow to a rate corresponding to the equivalent of braking under optimum conditions and thereby provides a pressure differential equivalent to the difference between the actual wheel velocity and the predetermined maximum wheel velocity during braking under optimum conditions. This pressure differential operates a control valve interposed between the apply valve and wheel brakes which exhausts brake pressure to release the brakes. A period of wheel acceleration follows the brake release and during the resulting wheel speed recovery the accumulator is recharged and the control valve shifts to recommunicate the apply valve to the brakes. An acceleration valve senses the wheel speed recovery permitted by the brake release and closes a hold valve which maintains the brake pressure at its then existing magnitude to provide the hold mode of the extremal cycle. When the wheel speed recovery ends, the acceleration valve and hold valve are shifted and the apply valve is recommunicated to the brakes to initiate the brake apply mode of the extremal cycle. A thermostatic valve disables the anti-lock control system when oil temperature is below a predetermined temperature at which fluid viscosity impairs system function. A cycle valve exercises the anti-lock system each time the transmission is shifted out of reverse into neutral or drive.</p>
