Peterbilt 579 Operator's Manual page 219

Unloading Operation
Always deflate air springs of the
auxiliary axles before attempting to
unload vehicle. This allows maximum
traction of the drive axles to control the
vehicle.
Non-liftable (Non-steerable) Axles
Some suspensions require dump valve
calibration.
Example: Neway dead axles do not
lift, but the air can be dumped out of
them to unload them when empty. Air
pressure is controlled via an adjustable
regulator. These axles need to be
calibrated for load.
Contact your authorized dealer or
axle/suspension manufacturer for
dump valve calibration procedures.
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