CDL Air Brakes TestInspecting Air Brake SystemsMedium
A driver is performing a pre-trip inspection on a vehicle with air brakes. They notice that when the parking brake is applied, the vehicle still rolls slightly on a level surface. What is the most likely cause?
- AThe low air warning device is faulty.
- BThe air compressor is not building enough pressure.
- CThe spring brakes are out of adjustment or worn.
- DThe service brakes are over-adjusted.
Show answer & explanationAnswer & explanation
Correct answer: C. The spring brakes are out of adjustment or worn.
The parking brake in an air brake system relies on spring brakes. If the vehicle rolls with the parking brake applied, it indicates that the spring brakes are not holding effectively, likely due to being out of adjustment, worn, or a mechanical issue preventing full application.
Why the other options are wrong
- A. A faulty low air warning device would not affect the mechanical application of the spring brakes.
- B. Insufficient air pressure build-up would lead to the spring brakes applying automatically at low pressure, not failing to hold when manually applied.
- D. Over-adjusted service brakes would cause drag, not a rolling vehicle when the parking brake is applied.
Parking Brake Effectiveness
The parking brake (spring brakes) must be capable of holding the vehicle stationary on a grade. Failure to do so indicates a problem with the spring brake system.
- Uses spring force to apply brakes.
- Tested by attempting to drive against the brake.
- Failure to hold points to adjustment issues or worn components.
Memory trick: Parking brake on, still rolls? Inspect the springs, before it tolls!