You have pushed the trailer air supply valve. When should you back a tractor under a trailer?

Step 8: Supply Air to the Trailer
• From the cab, push in the “air supply” knob or move the tractor protection valve control from the “emergency” to the “normal” position to supply air to the trailer brake system.
• Wait until the air pressure is normal.
• Check the brake system for crossed air lines.
- Shut the engine off so you can hear the brakes.
- Apply and release the trailer brakes and listen for the sound of trailer brakes being applied and released. You should hear the brakes move when applied and air escape when the brakes are released.
- Check the air brake system pressure gauge for signs of major air loss.
• When you are sure the trailer brakes are working, start the engine.
• Make sure the air pressure is up to normal.
[California Commercial Driver Handbook]