Which colour denotes the auxiliary line on a three-line braking system?

Explanation: If you're driving an articulated vehicle or a trailer combination, it's vital that you understand the rules that apply to coupling and uncoupling the brake lines. If you take a practical driving test with a trailer, you'll be expected to demonstrate this during the test. The lines are colour-coded: red is the emergency line, blue is the auxiliary line and yellow is the service line. They must be connected strictly in accordance with the correct procedure.

Category: Braking Systems (2021)

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