While running the doubles/triples air brake check, you walk back to the last trailer, crack open its shut-off valves, and hear no air escaping from either line. What should you check?

Explanation

Silence where you expect to hear air means the supply isn't making it all the way back, so what you check is whether every shut-off valve on the trailers and the dollies is actually open. The dollies are the part that's easy to overlook: a double or triple carries a shut-off valve where each trailer ends and often another on each dolly, so even one closed valve anywhere in that chain stops the air right there and starves every brake downstream of it. 49 CFR 383.111(b)(5) names procedures for conducting safe and accurate pre-trip inspections as a required air brake knowledge area, but it does not state this diagnostic step, so it is attributed to the manual. The wrong options either read silence as good news or chase the wrong system: the governor controls the compressor's cut-in and cut-out at the tractor rather than flow to the rear, an ABS lamp reports an electronic fault and says nothing about air reaching a trailer, and treating no escaping air as proof of a charged system reverses the test, since escaping air is the positive result being listened for.

Source: 49 CFR 383.111(b)(5)