When inspecting a doubles/triples combination, why must the driver check that the converter dolly air tank drain valve is closed?

Converter dolly air system: The dolly has its own air brake components connected into the combination's air system. The AIR TANK DRAIN VALVE (used to drain moisture from the tank) must be CLOSED during operation.

Why: An open drain valve leaks air, which could: reduce or prevent proper braking of the dolly and rear trailer; cause the air system to lose pressure; trigger automatic brake application or low-air warnings; INSPECTION of the dolly air system: Drain valve closed; air lines connected and not leaking, tangled, or dragging; air tank secure; brakes functioning.

Doubles/triples air system: More trailers means more air lines, connections, and components — each is a potential leak point; the driver must verify air flows to all trailers and all brakes work; check for air leaks at all the additional connections.

Leak test: Charge the system and check that all trailers' brakes apply and release properly; the extra coupling and air connections in doubles/triples create more inspection points; an open dolly drain valve is a specific, commonly tested item — it would compromise the braking of part of the combination.