of slip with the road is obtained i e that the speed of the previously locked wheel or wheels is increasing the braking force is 34 reapplied 34 when the wheel speed is sufficiently restored so that the braking force can be augmented in view of the suitable slip coefficient as is well known to those skilled in the art control for providing such signals are commercially available in a variety of electric control circuits the antilock system of the present invention operates as follows when a braking operation is to be by operator effected compressed air is introduced into higher pressure chamber 7 of pneumatic actuator 3 to move the power piston 5 rightwardly as seen in fig1 so that power screw shaft 1 is rotated forwardly by arm 2 to establish the desired braking force if at this time wheel lock is caused oil pressure is fed to the chamber 17 of the hydraulic cylinder mechanism 13 by the opening of first electromagnetic valve 21 and closing of the second electromagnetic valve 22 as a result an oil pressure force is directed leftwardly against piston 15 and in turn transmitted as a counteracting force to the power piston 3 connected to piston 15 by rod 16 the oil pressure force is caused to be sufficiently higher than the air pressure force acting on piston 5 so that the power screw shaft 1 is reversely rotated substantially instantaneously so that the braking force is abruptly decreased the extent of this drop depends upon the pressure and quantity of the oil fed to chamber 17 and may be suitably selected as desired if the wheel lock is released.