Air Brake System Operation
Before driving the vehicle, allow time for the air compressor to build up a minimum of 105 psi (724 kPa) pressure in both the primary and secondary systems. Monitor the air pressure system using the dual system air pressure gauge and the low-air-pressure telltale and buzzer. The warning telltale and buzzer deactivate when air pressure in both systems reaches approximately 70 psi (483 kPa).
Important: Before driving the vehicle, secure all loose items in the cab so that they will not fly forward during a full brake application. Make sure all passengers are wearing seat belts.
During normal brake stops, depress the brake pedal until braking action slows down the vehicle. Increase or decrease the pressure on the pedal so that the vehicle comes to a smooth, safe stop. Apply the parking brake when the vehicle is parked.
When parking a vehicle attached to a trailer that does not have spring parking brakes, apply the tractor parking brake. Chock the trailer tires before disconnecting the vehicle from the trailer.
Warning: If a trailer or combination vehicle is not equipped with spring parking brakes, do not park it by pulling only the trailer air supply switch. Apply the parking brake switch and secure the trailer before decoupling.
Notice: Never apply the service and spring parking brakes simultaneously. To do so transmits excessive input force on the brake components, which could damage or cause eventual failure of brake actuating components.
Note: The Electronic Air Processing Unit (EAPU) measures the air processed by the compressor and regenerates the air dryer as needed. During normal operation, the EAPU, generally once per fill cycle, regenerates the air dryer (purge) using system air. During high use, the EAPU may not use air for dryer regeneration and will have to complete multiple regenerations (purges) later. This may lead to purging noises while the vehicle is idle. These purges are part of normal operation and ensure air quality.
Brake Controls
If pressure in either air system drops below 70 psi (483 kPa), a buzzer sounds, and a ‘Low Air’ indicator activates. If the pressure in BOTH air systems drop to 20 to 45 psi (138 to 310 kPa), the tractor and trailer park brakes apply automatically, and the yellow parking brake switch and the red trailer air supply switch illuminates. See Fig. .
The switch with the yellow diamond-shaped icon in the control panel actuates the parking brake switch. See Fig. , item 3. Pulling the parking brake switch applies the tractor spring parking brakes. For the park brakes to be released, the parking brake switch must be pushed, the air pressure must be at least 100 psi (690 kPa), the keyswitch must be ON, and the service brake pedal must be pressed.
Note: When the vehicle is stopped, pulling the yellow brake switch activates both the tractor parking brakes and the trailer air supply. Pushing the yellow switch only releases the tractor parking brakes.
The switch with a red octagonal-shaped icon in the control panel actuates the trailer air supply. See Fig. , item 2. After the vehicle's air hoses are connected to a trailer and the pressure in both air systems is at least 100 psi (690 kPa) and the keyswitch is ON, the trailer air supply switch must be pushed. For some tractor and trailer configurations, it may take longer than 3 seconds for the trailer air tanks to fill. In such cases, press and hold the trailer air supply switch until the trailer is fully supplied. The LED indicator in the switch illuminates when the air is not supplied.
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Trailer Brake Lever
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Trailer Air Supply Switch (red octagon)
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Parking Brake Switch (yellow diamond)
Fig. 1, Dash-Mounted Brake Controls
Note: When driving without a trailer, the LED in the trailer air supply switch remains illuminated.
Notice: Do not use the spring parking brakes if the service brakes are hot, for example, after descending a steep grade. To do so could damage the brakes. Allow hot brakes to cool before using the spring parking brakes.
Do not use the spring parking brakes during freezing temperatures if the service brakes are wet. To do so could cause them to freeze. If the brakes are wet, drive the vehicle in low gear and lightly apply the brakes to heat and dry them.
Warning: Do not use the trailer service brakes for parking; they are not designed for this purpose. If air bleeds out of the trailer air tank during parking, the vehicle could roll, causing serious personal injury or property damage.
When the tractor and trailer parking brakes are both applied, the trailer parking brakes are released by pushing the trailer air supply switch. This leaves the tractor parking brake applied. Air pressure in the primary or secondary reservoir must be at least 100 psi (690 kPa) and the keyswitch must be on, before the spring parking brakes can be released.
Pull the trailer air supply switch to evacuate the trailer air before disconnecting a trailer.
Trailer Hand Brake
If equipped, the trailer brake lever is used for applying the trailer service brakes without applying the tractor service brakes. It is mounted on the right-hand control panel. See Fig. , item 1. The hand brake can be used for fifth wheel sliding or to check for air leaks in the trailer service line. The trailer brake only works while the trailer air supply is pressurized.
The valve can be partially or fully applied, but in any partially-on position, it will be overridden by a full application of the service brake pedal. Moving the lever down applies the trailer brakes; moving it up releases the trailer brakes.
Note: If there is a fault in the EBS, the trailer brake lever may not be functional.
Warning: Do not use the trailer service brakes for parking; they are not designed for this purpose. If air bleeds out of the trailer air tank during parking, the vehicle could roll, causing serious personal injury or property damage.
Important: Do not hang items on the trailer brake lever.
Automatic Slack Adjusters
Automatic slack adjusters should never be manually adjusted except during routine maintenance of the foundation brakes (such as replacing brake shoes), during slack adjuster installation, or in an emergency situation.
When the brake pushrod stroke exceeds the legal brake adjustment limit on a vehicle, there is likely a mechanical problem with the foundation brake components or the adjuster is improperly installed.
Visit a repair facility as soon as possible when brakes equipped with automatic slack adjusters are determined to be out of adjustment.
Warning: Manually adjusting an automatic slack adjuster to bring the pushrod stroke within legal limits is likely masking a mechanical problem. Adjustment is not repairing. Before adjusting an automatic slack adjuster, troubleshoot the foundation brake system and inspect it for worn or damaged components. Improperly maintaining the vehicle braking system may lead to brake failure, resulting in property damage, personal injury, or death.