Volvo VNL 780 Operator's Manual page 311

Trucks with automated Meritor Transmission: Drain the transmission air supply tank located
on the right-hand rail daily to check for contamination, water, oil, etc.
Charging Air to Another Vehicle
DANGER
Before working on a vehicle, set the
parking brakes, place the transmission in
neutral, and block the wheels. Failure to
do so can result in unexpected vehicle
movement and can cause serious personal
injury or death.
There are many methods which may be used to charge the air system of another vehicle.
Vehicles equipped with the standard two-valve system (trailer supply and parking brake)
must use the following procedure when using the emergency trailer air line as an auxiliary
air source, other than to charge the trailer air reservoirs.
1 Connect the emergency trailer air line to the auxiliary source.
2 Start the engine.
OPERATION 301