Your tractor unit has 3 air lines. You're connecting to a trailer that has 2 air-line couplings. What colour is the line you should not connect to the trailer?

Your tractor unit has 3 air lines. You're connecting to a trailer that has 2 air-line couplings. What colour is the line you should not connect to the trailer?

Blue

Correct answer

DVSA explanation

When connecting a three-line tractor unit to a two-line trailer, the blue line is the one that should not be connected to the trailer. This is the auxiliary line. Always follow the manufacturer's advice: it may be necessary to reconnect the extra line to the tractor unit.

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