When air pressure drops to between 20-40 psi, _________.

Emergency System Check/Fan Down:
- Turn electrical power on and begin fanning off the air pressure by rapidly applying and releasing the foot brake. Low air warning devices (buzzer, light, other) should activate before air pressure drops below 60 psi.
- Continue to fan off the air pressure. Normally within the range of 20-40 psi on a tractor-trailer combination vehicle, the tractor protection valve and parking brake valve should close (pop out). On other combination vehicle types and single vehicle types, the parking brake valve should close (pop out).