After the trailer has been coupled to the tractor, the tractor protection control valve should be placed in the ___ position.

1. In the open position to allow normal air flow.
2. In the closed position to prevent air from entering the trailer's brake system.
3. In the middle position for partial protection.
4. In the emergency position to lock the trailer brakes.

The tractor protection control valve should be in the open position after coupling the trailer to ensure normal air flow for the trailer's brake system to operate correctly.

Explanation:

After the trailer has been coupled to the tractor, the tractor protection control valve should be placed in the open position to allow normal air flow. This is essential to ensure that the trailer brakes operate correctly when the tractor brakes are applied. Positioning the valve correctly prevents accidental loss of air pressure which could release the trailer brakes unexpectedly, risking safety. It's important to note that the closed position is used to prevent air from entering the trailer's brake system, which is not the desired state after coupling. There is no middle or partial protection position for this control valve, and the emergency position immediately applies the trailer brakes and should not be used in normal coupling procedures.