brakes are wet, drive the vehicle in low gear and
lightly apply the brakes to heat and dry them.
If the trailer is not equipped with spring parking
brakes, pulling out the parking brake valve applies
the tractor parking brake and the trailer service
brakes. When the tractor and trailer parking brakes
(or trailer service brakes) are both applied, the trailer
brakes are released by pushing in the trailer air
supply valve, leaving the tractor parking brake
applied. Air pressure in the primary or secondary
reservoir must be at least 65 psi (448 kPa) before
the tractor spring parking brakes, or the trailer
service or spring parking brakes, can be released.
Automatic Slack Adjusters
Automatic slack adjusters should never be manually
adjusted except during routine maintenance of the
foundation brakes (e.g., replacing 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.
Engine Brake
Each engine manufacturer uses a specific engine
brake for their engine. Regardless of the
manufacturer, the engine brake is operated with the
principles outlined in here. Refer to the engine
manufacturer’s operation manual for details of their
particular engine brake product.
WARNING
To avoid injury from loss of vehicle control, do
not activate the engine brake system under the
following conditions:
•
on wet or slippery pavement, unless the
vehicle is equipped with antilock braking
system (ABS) and you have had prior
experience driving under these conditions;
•
when driving without a trailer or pulling an
empty trailer;
•
if the tractor drive wheels begin to lock, or
there is fishtail motion after the engine
brake is activated.
NOTE: When the engine brake is enabled, the
engine brake lamp illuminates in the ICU.
Whenever vehicle braking is required on good road
conditions, the engine brake may be used in
conjunction with the service brakes. There is no time
limit for operation of the engine brake. However, an
engine brake does not provide the precise control of,
and is not a substitute for, service brakes.
WARNING
Usage of the engine brake as the primary braking
system can cause unpredictable stopping
distances, that could result in personal injury or
property damage. Service brakes are the primary
vehicle braking system.
Since the engine brake is most effective at rated
engine speed, gear selection is very important.
Gearing down the vehicle within the limits of the
rated engine speed makes the engine brake more
effective. Recommended engine braking speed is
above 1800 rpm and below the rated speed.
"Control speed" is the speed at which the engine
brake performs 100 percent of the required downhill
braking, resulting in a constant speed of descent.
The control speed varies, depending on vehicle
weight and the downhill grade.
For faster descent, select a higher gear than that
used for control speed. Service brakes must then be
used intermittently to prevent engine overspeed and
to maintain desired vehicle speed.
For slower descent, select a lower gear, being
careful not to overspeed the engine. Occasional
deactivation of the engine brake may be necessary
Brake Systems
11.3
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