air pressure. This results in a different brake
pedal "feel," as the pedal seems to require more
travel and/or effort to slow or stop the vehicle.
However, the air brake proportioning system
actually improves vehicle control when the trac-
tor is in bobtail mode. When the tractor is tow-
ing a trailer, the rear brake chambers will re-
ceive full (normal) application air pressure.
When parking a vehicle attached to a trailer that
does not have spring parking brakes, apply the trac-
tor parking brakes. Chock the trailer tires before dis-
connecting the vehicle from the trailer.
WARNING
If a trailer or combination vehicle is not equipped
with spring parking brakes, do not park it by pull-
ing 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 puts excessive
input force on brake components, which could
damage or cause eventual failure of brake actuat-
ing components.
Brake 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
Fig. 12.1. The
valve can be partially or fully applied, but in any par-
tially on position it will be overridden by a full applica-
tion of the service brake pedal. Moving the lever
down applies the trailer brakes, moving it up releases
them. The lever will automatically return 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 octagon-shaped knob in the control panel
actuates the trailer air supply valve. See
Fig. 12.2.
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
pushed in. It should stay in to charge the trailer air
supply system and to release the trailer spring park-
ing brakes.
Pull the trailer air supply valve out before disconnect-
ing a trailer or when operating a vehicle without a
trailer. If pressure in both air systems drops to 35 to
45 psi (242 to 310 kPa), the trailer air supply valve
automatically pops out, exhausting the trailer air sup-
ply, and applying the trailer service or spring parking
brakes.
The yellow diamond-shaped knob in the control
panel actuates the parking brake valve. See
Fig. 12.2. Pulling out the parking brake valve applies
both the tractor and trailer spring parking brakes and
10/17/2001
f610591
Fig. 12.1, Trailer Brake Lever
f610291
1
2
03/10/99
1. Trailer Air Supply Valve
2. Parking Brake Valve
Fig. 12.2, Brake Valves
Brake System
12.2
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