At what general air pressure level does federal regulation require the low air pressure warning to activate?

Pick an answer

1. At 100 psi, the same as full system pressure
2. At or before 60 psi, or as specified by the manufacturer if lower
3. Exactly at 0 psi, once all air has been lost, far too late for the warning to give any useful notice
4. Only after the parking brake has already been manually released

Correct!

Not quite. The correct answer is highlighted.