CDL Combination Vehicles TestCombination Vehicle Air BrakesHard
A combination vehicle has an air leak in the emergency (supply) air line between the tractor and the trailer. What will be the primary effect on the trailer's braking system if this leak occurs while driving?
- AThe trailer ABS system will activate continuously, causing pulsing.
- BThe trailer service brakes will apply automatically and gradually.
- CThe trailer service brakes will become unresponsive due to loss of pressure.
- DThe trailer emergency (spring) brakes will apply automatically.
Show answer & explanationAnswer & explanation
Correct answer: D. The trailer emergency (spring) brakes will apply automatically.
The emergency (supply) air line supplies air to the trailer's air tanks and controls the release of the trailer's spring brakes. A loss of pressure in this line, such as from a leak, will cause the trailer's emergency (spring) brakes to apply automatically as a safety feature. This is often referred to as a 'trailer breakaway' situation.
Why the other options are wrong
- A. ABS activates during hard braking to prevent wheel lock-up, not in response to a primary air supply line leak.
- B. The service brakes are controlled by the service line, not directly by the emergency line pressure loss in this manner.
- C. While the service brakes might eventually be affected by total air loss, the immediate and primary effect of an emergency line leak is the application of spring brakes.
Emergency Air Line Leak (Combination Vehicle)
A critical failure in the air line that supplies air to the trailer's air tanks and controls its spring brakes. A leak in this line will cause the trailer's spring brakes to automatically apply, resulting in an emergency stop.
- Emergency line (red) supplies air to trailer air tanks.
- Maintains pressure to keep spring brakes released.
- Leak causes automatic application of trailer spring brakes.
- Requires immediate, controlled stop by the driver.
Memory trick: Red = Release, Blue = Brake.