CDL Combination Vehicles Test
Combination vehicle air brakes: practice questions
Our CDL Combination bank has 48 questions on this topic. Here are 5 of them with answers; the full mock exam draws from every topic, like the real one.
Question 1
Trailers without spring brakes have no parking brake. When do their emergency brakes come on?
- When the air supply knob is pulled out or the trailer is disconnected
- Only when the brake pedal is pressed hard enough to lock the wheels
- Only when the hand valve is applied
- Only when the ABS lamp is on
Show the answer
A. When the air supply knob is pulled out or the trailer is disconnected — These trailers have no parking brake; the emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected (CDL Manual 6.2.8)
Question 2
What color is the emergency (supply) line often coded?
- Blue
- Yellow
- Green
- Red
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D. Red — Emergency lines are often coded red; the service line is blue (CDL Manual 6.2.4, 6.2.5)
Question 3
What color is the service line usually coded?
- Red
- Yellow
- Blue
- Black
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C. Blue — Blue is used for the service lines and red for the emergency (supply) lines (CDL Manual 6.2.5)
Question 4
What does the tractor protection valve do?
- Sends extra air to the trailer brakes when the trailer is heavily loaded
- Controls when the compressor pumps air
- Limits air to the steering axle on ice
- Keeps air in the tractor if the trailer breaks away or leaks badly
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D. Keeps air in the tractor if the trailer breaks away or leaks badly — The tractor protection valve keeps air in the tractor or truck brake system should the trailer break away or develop a bad leak (CDL Manual 6.2.2)
Question 5
Why are crossed air lines especially dangerous on an older trailer without spring brakes?
- The trailer brakes would lock on permanently
- You could drive away with no trailer brakes
- The tractor brakes would stop working
- The trailer would uncouple at speed
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B. You could drive away with no trailer brakes — On older trailers without spring brakes, with crossed lines you could drive away but you wouldn't have trailer brakes (CDL Manual 6.2.5)