Air Brakes on Combination Vehicles
23 free questions, part of the Combination Vehicles CDL practice test
Air brakes on a combination vehicle run on a two-line system: the service line carries the signal to apply the brakes, and the emergency line keeps the trailer's spring brakes released and sets them if the trailer loses air or breaks away. These questions cover the trailer hand valve, the tractor protection valve, glad hands, shut-off cocks, and what goes wrong if you cross the air lines when coupling. ELDT.io's free bank has 23 air brake questions on combination vehicles, grounded in 49 CFR 383.111(c)(3)(ii) and the AAMVA CDL Manual.
This topic has 23 questions, split into 2 tests of 11-12 questions each:
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Start Class A CDL Theory ($15)Questions about Air Brakes on Combination Vehicles
What is the trailer hand valve (trolley valve) actually for?
It is meant for testing trailer brakes, not for normal driving and not for holding the vehicle while parked. Using it during normal driving upsets the balance between tractor and trailer braking and raises jackknife risk.
What happens if I cross the service and emergency air lines when coupling?
On a trailer with spring brakes, crossed lines mean the trailer brakes will not release when you supply air, so you find out right at the hookup. On an older trailer without spring brakes, crossed lines can let the trailer roll with no brakes at all, which is far more dangerous.
What color are the glad hands, and why does it matter?
Blue marks the service line and red marks the emergency line. Matching the colors when you connect the glad hands is what prevents the crossed-line problem above.