freightliner M2 Plus 2023 Manual page 145

However, the air brake proportioning system
actually improves vehicle control when the
tractor is in bobtail mode. When the tractor is
towing a trailer, the rear brake chambers receive
full (normal) application air pressure.
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 out only the trailer air supply valve knob.
This would apply only the trailer service brakes.
If air were to bleed from the trailer brake system,
the trailer brakes would release, possibly causing
an unattended runaway vehicle.
NOTICE
Never apply the service and spring parking
brakes simultaneously. To do so transmits
excessive input force to the brake components,
which could damage or cause eventual failure of
brake actuating components.
Brake Hand Controls
The trailer brake lever (hand control valve) is used
for applying the trailer brakes without applying the
truck or tractor service brakes. It is usually mounted
on the right-hand control panel. See
valve can be partially or fully applied, but in any
partially applied position, it is overridden by a full
application of the brake pedal. Moving the lever
down applies the trailer brakes, while moving it up
releases the trailer brakes. The lever automatically
returns to the up position when it is released.
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.
The red octagonal-shaped knob in the control panel
actuates the trailer air supply valve. See
After the vehicle's air hoses are connected to a
trailer and the pressure in both air systems is at least
65 psi (448 kPa), the trailer air supply valve must be
11/
pushed in. It should stay in to charge the trailer air
supply system and to release the trailer spring
parking brakes.
Pull the trailer air supply valve out before
disconnecting a trailer or when operating a vehicle
without a trailer.
If pressure in both air systems drops to 20 to 45 psi
(138 to 310 kPa), the red trailer air supply valve and
yellow parking brake valve automatically pop out,
applying both the tractor and trailer spring parking
brakes. If the trailer is not equipped with spring
parking brakes, the trailer service brakes are applied.
Fig. 11.1. The
The yellow diamond-shaped knob in the control
panel actuates the parking brake valve. See
Fig.
both the tractor and trailer spring parking brakes and
automatically causes the trailer air supply valve to
pop out. Pushing in the parking brake valve releases
the tractor parking brakes.
Do not use the spring parking brakes if the
service brakes are hot, such as 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
Fig. 11.1.
freezing temperatures if the service brakes are
wet. To do so could cause them to freeze. If the
2
1
29 /20 22
1. Parking Brake Control
2. Trailer Air Supply Control
3. Hazard Warning Switch
4. Trailer Brake Control
Fig. 11.1, Right-Hand Control Panel
11.1. Pulling out the parking brake valve applies
NOTICE
Brake Systems
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3
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11.2