A dual air brake system has two separate air brake circuits, both controlled by the same set of brake controls. Each circuit has its own air tanks, hoses, lines, etc. The primary circuit typically operates the regular brakes on the rear axle or axles. The secondary circuit operates the regular brakes on the front axle (and possibly one rear axle). If one circuit loses air pressure, the other circuit should still work as long as the compressor and governor are still working properly.