What is special about checking the air system on a doubles combination?

Doubles air systems require special inspection because air must travel through multiple connections: tractor to lead trailer via standard glad hands; lead trailer to dolly via another set of glad hands; dolly to rear trailer via a third set. Each connection is a potential leak point. The air leakage rate test (engine off, brakes released, time the pressure drop for 1 minute) for a doubles combination has a higher allowable rate than a single combination — 3 psi per minute for combinations with one or more trailers, or 4 psi per minute with brakes applied — to account for the additional connections. Leakage above the limit means a leak somewhere in the system that must be found and repaired.