During the "check that air flows to all trailers" test on a combination vehicle, you charge the system, go to the rear, and open the emergency line shut-off valve at the back of the last trailer — but you hear no air escaping. What should you do?

AAMVA CDL Manual, Section 6.5.2, p. 6-14 ("Check That Air Flows to All Trailers")

If a shut-off valve is left closed somewhere in the combination, air never reaches the rearmost trailer's brakes, and that trailer will have no service or emergency braking at all even though the tractor's own air gauges look completely normal. This step is the only part of the pre-trip check that can catch a mid-combination blockage invisible from the cab, so skipping it can send a rig onto the road with an entire trailer's brakes disabled.