AIR SYSTEM (Con't).
The dual air brake system consists of two independent air brake systems that
use a single set of brake controls. Each system has its own reservoirs, plumbing, and brake
chambers. The primary system operates the service brakes on the rear axle; the secondary
system operates the service brakes on the front axle. On tractor-trailer configurations, service
brake signals from both systems are sent to the trailer.
Loss of air pressure in the primary system causes the rear service brakes to
become inoperative; front brakes will continue to be operated by secondary system air
pressure. In addition, trailer brakes will be operated by the secondary system. Loss of
secondary system air pressure causes the front axle brakes to become inoperative; rear
service brakes and trailer brakes will be operated by the primary system.