Peterbilt Medium Duty Truck Owner's Manual - Peterbilt Motors

STARTING AND OPERATING THE VEHICLE
WARNING! Do not use an engine retarder
when you are driving bobtail or with an
unloaded trailer. Using engine retarders while
bobtailing or with an unloaded trailer can
cause a wheel lockup resulting in less control
and/or jackknife. The trailer may not load the
rear tractor tires enough to provide necessary
traction. When you are bobtail or unloaded,
you can have a serious accident, causing
death or personal injury, if your wheels lock
suddenly during braking.
Bobtail Brake Proportioning System
When a trailer is not connected, the drive axle brake application
pressure will automatically be limited by the proportioning
system.
The brake proportioning system regulates the application
pressure to the rear drive axle. To provide equivalent braking
power, tractors (driven bobtail) will require greater
brake pedal application than other types of vehicles not
equipped with a proportioning system.
USING THE BRAKE SYSTEM
On tractors that do not have anti-lock brake systems
(ABS) a bobtail brake proportioning system is installed.
When a trailer is not connected (bobtail mode), the brake
application pressure (on the rear drive axle) will automatically
be limited by the proportioning system.
Engine Retarder (option)
An engine retarder or exhaust brake may be installed (as
an option) to create a braking effect on the drive wheels.
This device uses your engine's power to slow your vehicle
down. Because it can help keep your vehicle's brakes
from overheating, it can save wear and tear on the service
brakes. However, the retarder is not an emergency brake.
Ideally (on normal road surfaces), you should slow your
vehicle with the retarder (where permitted by law) and use
the service brakes only for stopping completely. Operating
this way will greatly prolong the life of the brakes.
Medium Duty (R05/09) Y53-6027 – 117 –