CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
A driver is performing a functional check of the trailer spring brakes on a combination vehicle. After charging the trailer air system, the driver pushes in the trailer air supply (red) knob and then pulls it out. What should happen when the red knob is pulled out?
- AThe tractor parking brakes should release.
- BThe trailer service brakes should apply.
- CThe trailer spring brakes should apply.
- DThe trailer air tanks should begin to drain.
Show answer & explanationAnswer & explanation
Correct answer: C. The trailer spring brakes should apply.
Pulling out the trailer air supply (red) knob releases air pressure from the emergency (red) air line and the trailer's air tanks, causing the trailer's spring brakes to apply. This is a critical safety feature.
Why the other options are wrong
- A. The tractor parking brakes are typically controlled by a separate yellow knob or lever in the cab.
- B. The trailer service brakes are applied by the foot pedal or hand valve (blue line), not by pulling out the red knob.
- D. While pulling the knob out does cause air to exhaust, the primary and intended functional effect is the application of the spring brakes, not simply draining the tanks.
Trailer Air Supply Knob (Red) - Apply Spring Brakes
Pulling out the red trailer air supply knob removes air pressure from the trailer's emergency line, causing the trailer's spring (parking/emergency) brakes to apply.
- Controls trailer emergency/parking brakes.
- Push in to release, pull out to apply.
- A critical safety control.
Memory trick: Red knob out, trailer fixed to the ground, no doubt.