Final answer:
The tractor protection control valve should be placed in the normal position after the trailer has been coupled to the tractor to allow normal operation of the brake system.
Step-by-step explanation:
After the trailer has been coupled to the tractor, the tractor protection control valve should be placed in the normal position.
The tractor protection control valve is designed to protect the tractor's air supply system. When the trailer is coupled to the tractor, the valve should be set to the normal position to allow normal operation of the tractor's brake system.
Setting the valve to the normal position ensures that the air supply is properly connected to the tractor's brake system, allowing the driver to control the trailer's brakes using the tractor's brake pedal.