Abstract: A heavy-duty disc brake assembly and pneumatic actuating assembly therefor. The pneumatic actuating assembly may include a pair of annular pistons operatively connected for simultaneous movement thereof, apparatus being provided for supplying air under pressure to the working faces thereof, or a single piston spring biased to braking position with apparatus for supplying air under pressure to either face thereof. The disc brake assembly includes a plurality of relatively movable rotor and stator discs, and preferably a closed cooling system with circulatory loop is provided.