CDL General Knowledge TestCombination Vehicles (if applicable)Medium
What is the primary function of the 'emergency' air line (red glad hand) in a combination vehicle air brake system?
- ATo activate the trailer's ABS system during hard braking.
- BTo release the trailer's parking brakes and supply air to the trailer's air tanks.
- CTo supply air to the trailer's service brakes for normal stopping.
- DTo provide a secondary air supply in case of a service line failure.
Show answer & explanationAnswer & explanation
Correct answer: B. To release the trailer's parking brakes and supply air to the trailer's air tanks.
The emergency (supply) line supplies air to the trailer's air tanks and holds the trailer's spring (parking) brakes in the released position. A loss of pressure in this line will cause the trailer's emergency brakes to engage.
Why the other options are wrong
- A. The ABS system is activated by the service line and its own sensors, not directly by the emergency line.
- C. This is the function of the service (blue glad hand) air line.
- D. While it's a 'supply' line, its primary function is not a secondary supply for service brakes, but rather to ensure the trailer's air system is charged and parking brakes are released.
Emergency Air Line Function
The red glad hand line that supplies air to the trailer's air tanks and releases its spring brakes.
- Also known as the supply line.
- Loss of pressure applies trailer brakes.
- Connects to the trailer's air reservoir.
Memory trick: Red for release, blue for the squeeze.