In a dual air brake system, one system will typically control the rear brakes, and the other _________.

Dual Air Brake Systems: Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer if there is one. The first system is called the 'primary' system. The other is called the 'secondary' system.