Abstract: A vehicle braking system is provided having alternate, automatically selected actuating fluid circuits, either hydraulic or air circuits, interconnected between a master brake cylinder actuated by a vehicle operator and a brake actuating cylinder that operates the brake shoes at a wheel of the vehicle. A first actuating fluid circuit is normally connected in communicating relationship between the master cylinder and actuating cylinders by fluid-pressure responsive valves. Failure in buildup of fluid pressure in the first circuit when the master cylinder is actuated, such as would be due to a break or leak in the primary actuating fluid line, results in automatic operation of the valves to interconnect a second circuit in bypass relationship to the first circuit. The pressure responsive valves are not operational unless the brake master cylinder is being actuated.