At what air pressure must a low air pressure warning device activate?

Pick an answer

1. Only after the spring brakes have already activated
2. Below 60 psi, or when air pressure drops to 50 percent of the compressor's governor cut-out pressure, whichever is higher
3. Only when the parking brake is applied
4. Below 100 psi, or when air pressure drops to 25 percent of the compressor's cut-out pressure, whichever is higher, though most drivers assume it's a single fixed number

Correct!

Not quite. The correct answer is highlighted.