You are coupling a tractor to a semitrailer and have backed up, but are not under it. Before backing under, what should you hook up?

Step 7: Connect Air Lines to the Trailer
• Check the glad hand seals and connect the tractor emergency air line to the trailer emergency glad hand.
• Check the glad hand seals and connect the tractor service air line to the trailer service glad hand.
• Make sure the air lines are safely supported where they will not be crushed or caught while the tractor is backing under the trailer.
[California Commercial Driver Handbook]