CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
You are performing a test of the trailer spring brakes on a combination vehicle. After charging the trailer air tanks, you push in the red 'emergency' dashboard knob. What should happen to the trailer brakes?
- AThe trailer service brakes should release, allowing the vehicle to roll.
- BThe trailer spring brakes should apply, and the vehicle should hold.
- CThe trailer service brakes should apply, and the vehicle should hold.
- DThe trailer spring brakes should release, allowing the vehicle to roll.
Show answer & explanationAnswer & explanation
Correct answer: D. The trailer spring brakes should release, allowing the vehicle to roll.
Pushing in the red 'emergency' dashboard knob (often labeled 'Trailer Air Supply' or 'Tractor Protection Valve') sends air pressure to the trailer's emergency line, which in turn releases the trailer's spring brakes. When this knob is pulled out, it exhausts air from the emergency line, causing the spring brakes to apply.
Why the other options are wrong
- A. The service brakes are controlled by the foot brake or hand valve, not the red emergency knob. Pushing the red knob releases the spring brakes, allowing the vehicle to roll if not held by other brakes.
- B. Pushing the red knob releases the spring brakes, it does not apply them.
- C. Pushing the red knob releases the spring brakes, it does not apply the service brakes.
Trailer Air Supply (Red) Knob Function
The red, octagonal dashboard control knob (often labeled 'Emergency' or 'Trailer Air Supply') that controls the air supply to the trailer's emergency line. Pushing it in supplies air to release the trailer spring brakes; pulling it out exhausts air to apply them.
- Red, octagonal knob.
- Controls trailer emergency air supply.
- Push in to release trailer spring brakes.
- Pull out to apply trailer spring brakes (emergency/parking).
Memory trick: Red Knob Releases, Pulling Parks.