Volvo VNL 780 Operator's Manual page 325

On vehicles equipped with the standard two valve system, the operation of one valve
together with the other permits the operator to select the desired functions described below:
Red Valve (Trailer Air
Supply)
Out
In
In
Out
Parking Brake Release
VN and VHD models have engines equipped with engine electronic control units (EECU).
These have two features that may impact the brake system. One is the anti-stall device in the
EECU which senses torque demand and automatically controls the engine to compensate
and maintain speed. On tractors equipped with parking brakes on only one axle, the anti-stall
device keeps the engine from stalling, allowing the driver to pull away with the brakes
dragging. This may result in overheated brakes and damaged wheel hubs.
WARNING
Continual dragging of the brakes will
cause brakes to overheat. This may lead
to wheel end fire. To avoid overheating,
make sure both parking brake knobs are
pushed in before driving off.
If the yellow knob is pulled out and the red knob is pushed in, a condition is created where
the trailer parking brakes are not applied and only the single drive axle brakes are applied. In
this condition, the vehicle can be driven away. Therefore, it is very important that the driver
make sure the yellow AND red knobs are pushed in all the way before driving the vehicle.
Yellow Valve (System Park)
Out
Out
In
In
OPERATION
Function (Mode)
System Park
Trailer Charge
Normal Running
Bobtail
315