Your tractor unit has three air lines. You’re connecting to a trailer that has two air-line couplings. What colour is the line you shouldn’t connect to the trailer?

Explanation: When connecting a three-line tractor unit to a two-line trailer, the blue line is the one that shouldn't 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.

Category: Braking Systems

References: The Official DVSA Guide to Driving Goods Vehicles: Section 2