Abstract: Anti-lock system for the air brakes of wheeled vehicles to be interposed between the existing relay valves and air-actuated diaphragms for applying the brakes and to be actuatable alternately in a pulsing sequence by a pulse-generating circuit after a predetermined electrical time delay sufficient to allow full frictional brake engagement upon actuation by the operator of the air brakes so as to block air flow from the relay valves to the air-actuated diaphragms and to vent air from the air-actuated diaphragms, thereby causing momentary release of the brakes, and then to re-establish air flow from the relay valves to the air-actuated diaphragms and to block venting of air from the air-actuated diaphragms, thereby causing a re-application of the brakes; and a solenoid-controlled valve for the anti-lock system having passageways of critical length and diameter.