You are coupling a set of doubles, and the second trailer has no spring brakes of its own. How should it be secured before you couple it?

Explanation

The procedure is to pull the tractor up close, hook up the emergency line, let the trailer's air tank charge, then unhook that line, which sets the trailer's emergency brakes as long as the slack adjusters are properly adjusted, and to chock the wheels if there is any doubt about the brakes. The logic is that charging the tank and then cutting the supply is what makes the trailer's own stored air apply its brakes, so the trailer holds itself without the tractor staying connected to it. 49 CFR 383.115(a) names procedures for assembly and hookup of the units as a required knowledge area for this endorsement, but it does not state this sequence, so the procedure is attributed to the manual. The wrong options all leave the trailer relying on something that is not there: a trailer without spring brakes has no parking brake control to set, leaving it unsecured is the hazard the procedure exists to prevent, and holding the hand valve would require the driver to stay in the cab at the moment the work is happening outside the vehicle.

Source: 49 CFR 383.115(a)