CDL Air Brakes TestAir Brake System PartsMedium
A driver is performing a pre-trip inspection on a commercial vehicle with a dual air brake system. They observe that when the parking brake is set, the spring brakes activate, but the parking brake control knob does not 'pop out' automatically. What does this indicate?
- AThe spring brakes are improperly adjusted.
- BA possible malfunction of the parking brake control valve.
- CThe low air pressure warning system is faulty.
- DA normal operation for some older air brake systems.
Show answer & explanationAnswer & explanation
Correct answer: B. A possible malfunction of the parking brake control valve.
In a properly functioning air brake system, when the parking brake is set (air pressure is released from the spring brakes), the parking brake control knob should 'pop out' automatically. If it doesn't, it indicates a malfunction of the parking brake control valve.
Why the other options are wrong
- A. Improperly adjusted spring brakes would affect braking effectiveness, not the mechanical action of the control knob.
- C. The low air pressure warning system is distinct from the mechanical operation of the parking brake control knob.
- D. Automatic pop-out is a standard safety feature, not an age-dependent variable.
Parking Brake Control Valve
A valve that controls the air pressure to the spring brakes, applying or releasing the parking brake.
- Typically a push-pull knob on the dashboard.
- Pushed in to release parking brake (charges spring brakes with air).
- Pulled out to set parking brake (releases air from spring brakes, allowing them to engage).
- Should automatically 'pop out' when air pressure drops below a critical level (20-45 psi).
Memory trick: Pull to Park, Push to Go, Pop-out says 'Stop, no go!'