Brakes Air Brake System General Information Before driving your vehicle, allow time for the air com- pressor to build up a minimum of 95 psi (655 kPa) pres- sure in both the primary and secondary systems. Moni- tor the air pressure system by observing the dual system air pressure gauge and the low-air-pressure warning light and buzzer. The warning light and buzzer shut off when air pressure in both systems reaches 64 to 76 psi (441 to 524 kPa). A dual air brake system consists of two independent air brake systems which use a single set of brake controls. Each system has its own reservoir, 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. Service brake sig- nals from both systems are sent to the trailer. The warning light and buzzer come on if air pressure drops below 64 to 76 psi (441 to 524 kPa) in either sys- tem. If this happens, check the air system pressure gauges to determine which system has low air pres- sure. Although the vehicle’s speed can be reduced us- ing the foot brake control pedal, either the front or rear service brakes will not be operating, causing a longer stopping distance. Bring the vehicle to a safe stop, and have the air system repaired before continuing.
WARNING Do not operate the vehicle with the front brakes backed off or disconnected. Backing off or discon- necting the front brakes will not improve vehicle handling and may lead to loss of vehicle control resulting in property damage or personal injury. WARNING 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. Do not drive the vehicle with the parking brakes caged.
If the vehicle is driven with the parking brakes caged, there would be no means of stopping the vehicle if a complete loss of air pressure oc- curred. This could result in serious personal injury or vehicle damage. Before caging the spring park- ing brakes, make the connection to a towing vehicle or chock the tires. 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 sys- tem. 7.1