CDL air brakes practice

You continue the Step 7 check on a straight truck by fanning the brake pedal after the low air warning comes on. The yellow parking brake knob pops out when the gauge reads 30 psi. How should you interpret this?

• AIt is a defect, because the knob should pop out at 60 psi.
• BIt is a defect, because the knob should stay in until 0 psi.
• CIt is normal only if the vehicle has a single air system.
• DIt is normal, because spring brakes apply between 20 and 45 psi.