As part of a pre-trip air brake check, why should you also test that the tractor protection valve closes properly?

Pick an answer

1. It only affects trailer lighting, not any part of the braking system
2. It has already been tested once at the factory and never needs checking again, when components can wear or stick over years of service regardless of how they performed when new
3. The tractor protection valve is only relevant during trailer hookup, not while driving
4. So you know the tractor's own air will be protected if the trailer connection fails later during the trip

Correct!

Not quite. The correct answer is highlighted.