Combination Vehicles · Combination Vehicle Air Brakes
A combination vehicle is towing two trailers. According to the manual, how should the air-line shut-off valves (cut-out cocks) at the back of each trailer be set?
1.Open at the back of every trailer in the combination except the very last one, which must be closedCorrect answer
2.Closed on every single trailer in the combination, since shut-off valves are only ever used when parked
3.Open only on the last trailer in the entire combination, and closed on all of the others
4.Closed at the front of each trailer and left wide open at the rear of each one instead
Explanation
Shut-off valves let a driver close the air-lines when another trailer isn't being towed. Every shut-off valve stays open so air can reach the whole combination; the exception is the rearmost trailer's rear valve, which is closed to seal the end of the line. This is manual-only content, topic-scoped to 49 CFR 383.111(c)(3)(ii). Closing every valve would stop air flow to any of the trailers, and opening only the last trailer's valve reverses which end is actually closed. The front/rear split described in the fourth option doesn't match how the manual frames the rule, which is specifically about the last trailer's rear valve.
Source: 49 CFR 383.111(c)(3)(ii)
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