Abstract: A system for braking axle-attached wheels of a tractor trailer and the like includes first and second arrays of discs symmetrically opposed about the centerline of a tubular axle containing the arrays of discs; symmetrical also in each array of discs is a first plurality of discs slidably attached to the axle and a second plurality of discs slidably attached to the housing; the axle comprises a stub axle at each end protruding from the housing in position for mounting a wheel or wheels; in braking, a two-lobe, cam-actuated system simultaneously compresses the respective arrays of discs in directions opposite each other for frictional retardation of the axle; bearings carry the axles; the cam actuated system includes a spreader plate driving a pressure plate on each side of centerline.