CDL Air Brakes TestAir Brake System PartsEasy
During an air brake system inspection, a driver observes that the parking brake control knob is 'pulled out'. What does this indicate about the vehicle's braking status?
- AThe anti-lock braking system (ABS) is active.
- BThe spring brakes are engaged.
- CThe service brakes are fully applied.
- DThe front brakes are disengaged.
Show answer & explanationAnswer & explanation
Correct answer: B. The spring brakes are engaged.
In commercial vehicles with air brakes, pulling out the parking brake control knob releases air pressure to the spring brakes, allowing the springs to apply the parking/emergency brakes. Pushing the knob in charges the spring brakes with air, releasing them.
Why the other options are wrong
- A. ABS activation is an automatic function during hard braking and is indicated by a light, not the parking brake knob.
- C. The service brakes are applied by depressing the foot pedal, not by the parking brake knob.
- D. The parking brake knob controls the rear (and sometimes front) spring brakes, but 'pulled out' means they are engaged, not disengaged.
Parking Brake Control Knob
The parking brake control knob in an air brake system controls the application and release of the spring brakes, which function as parking and emergency brakes.
- Pulled out = spring brakes engaged (parking/emergency).
- Pushed in = spring brakes released (air pressure holds them off).
- Directly controls air pressure to spring brake chambers.
Memory trick: Pull to park, push to go.