Abstract: An apparatus is set forth for utilization with tractor-trailer organizations wherein a rotor is mounted fixedly and rotatably with a rear axle extending longitudinally and exteriorly of a rear axle housing, wherein the rotor includes a plurality of callipers. The callipers are actuated by a master cylinder positioned adjacent the rear axle and mounted on the axle housing. A torsion bar transmits rotative force from a remotely positioned pneumatic booster cylinder to the master cylinder utilizing cam collars mounted on the torsion bar for the master cylinder and pneumatic booster assembly.