CDL Air Brakes TestAir Brake System PartsHard
A technician is performing a pre-trip inspection on a commercial vehicle. When testing the spring brakes, what action should the technician take to ensure they are functioning correctly?
- AEngage the front brake limiting valve to 'slippery' and then apply the service brakes.
- BDepress the brake pedal firmly and listen for air leaks.
- CApply the parking brake and check if the vehicle moves when placed in gear.
- DDrain all air from the tanks and verify the parking brake engages automatically.
Show answer & explanationAnswer & explanation
Correct answer: D. Drain all air from the tanks and verify the parking brake engages automatically.
Spring brakes are designed to engage automatically when air pressure drops to a critically low level (typically 20-45 psi). Draining the air tanks simulates this condition, allowing the technician to verify that the spring brakes (parking/emergency brakes) engage as intended.
Why the other options are wrong
- A. The front brake limiting valve is for service brakes on slippery surfaces and is unrelated to testing spring brake automatic engagement.
- B. Depressing the brake pedal checks service brake function and leaks, not the automatic engagement of spring brakes.
- C. While this checks if the parking brake holds, it doesn't specifically confirm the spring brake's automatic engagement feature upon air loss.
Spring Brake Automatic Engagement
Spring brakes are designed to engage automatically when air pressure falls below a critical level, serving as both parking and emergency brakes.
- Engage at 20-45 psi.
- Released by air pressure, applied by spring force.
- Must be tested by draining air to verify automatic application.
Memory trick: Spring brakes: no air, they're there!