CDL Air Brakes TestInspecting Air Brake SystemsMedium
A driver is performing a pre-trip inspection and observes that the spring brakes on the vehicle do not apply automatically when the air pressure drops below 20 psi. What is the most likely cause of this issue?
- AThe spring brake chambers are improperly adjusted.
- BThe parking brake control valve is malfunctioning.
- CThe air compressor is not building enough pressure.
- DThe low air pressure warning system is faulty.
Show answer & explanationAnswer & explanation
Correct answer: B. The parking brake control valve is malfunctioning.
Spring brakes are designed to apply automatically when the air pressure in the system drops to a critical level (typically 20-45 psi, often below 20 psi for full application). If they fail to apply, the parking brake control valve (often integrated with the parking brake dash button/knob) is the most likely culprit, as it controls the release and application of the spring brakes.
Why the other options are wrong
- A. Improper adjustment of the spring brake chambers would affect braking effectiveness, but not prevent the automatic application of the spring brakes due to low air pressure.
- C. While the compressor might not be building enough pressure, the question states the pressure has already dropped below 20 psi, so the issue is with the application, not the pressure drop itself.
- D. A faulty low air warning system would mean no warning, but the spring brakes should still apply automatically at low pressure.
Spring Brake Automatic Application
Spring brakes are designed to apply automatically when the air pressure in the brake system falls below a critical threshold (typically 20-45 psi), serving as an emergency and parking brake.
- Safety feature for low air pressure.
- Activates automatically.
- Controlled by the parking brake control valve.
Memory trick: Spring brakes stuck? Check the valve, it's the control, not the air itself.