CDL General Knowledge TestAir Brakes (if applicable)Easy
A commercial vehicle is equipped with a spring parking brake. If the air pressure in the system drops below approximately 20-45 psi, what will happen automatically?
- AThe low air pressure warning light will activate.
- BThe air compressor will automatically shut off.
- CThe spring parking brakes will automatically apply.
- DThe service brakes will engage partially.
Show answer & explanationAnswer & explanation
Correct answer: C. The spring parking brakes will automatically apply.
Spring parking brakes are designed to apply automatically (springs push the brakes on) when air pressure drops to a critically low level (typically between 20 and 45 psi) to ensure the vehicle stops or remains stopped.
Why the other options are wrong
- A. The low air pressure warning light activates at a higher pressure (usually around 60 psi) to warn the driver before the spring brakes apply.
- B. The air compressor is controlled by the governor and would continue to try and build pressure unless it's faulty.
- D. The service brakes rely on air pressure to apply, so they would become less effective, not partially engage automatically.
Spring Parking Brakes
Brakes that use strong springs to hold them in the 'on' position; air pressure is used to hold them 'off'.
- Apply automatically when air pressure drops too low.
- Used for parking and as an emergency brake.
- Release only when sufficient air pressure is built.
Memory trick: Low Air? Springs Say 'STOP!'