During the air-flow check, if a driver does NOT hear air escaping from both lines at the rear of the last trailer, what should be checked?

49 CFR 383.115(e); AAMVA CDL Manual Section 7.4.1, p. 7-4

A closed shut-off valve anywhere in the chain is the single most common reason air fails to reach the back of a doubles/triples rig, and it's a problem a driver can usually fix on the spot by locating and opening the valve rather than needing a mechanic.