How should a driver test that the trailer brakes work on a doubles or triples combination before driving?

Before driving, the driver builds the air system to normal pressure and verifies the system works: the low-air warning operates, pressure builds within the proper time, and the brakes apply and release correctly. For the trailers, the driver confirms the air lines are connected and the shut-off valves are set so air reaches all trailer brakes, then performs brake tests — including a gentle tug test at low speed — to confirm the trailers are securely coupled and that their brakes hold. Because a doubles or triples set has multiple trailers and dollies, a missed brake connection on a rear trailer could leave part of the combination without brakes.