Braking systems, air brakes and coupling checks
Air brakes are where a small oversight becomes a runaway vehicle. The test asks both about the system - reservoirs, lines, spring brakes, EBS - and about the coupling routine that proves the trailer is actually attached and braked.
How the system works
- A compressor charges reservoirs through an air dryer; the dual-circuit system keeps a backup circuit if one fails.
- Coupling lines: red is the supply line (roughly 8.5 bar), yellow is the service/brake line, and the black ISO 7638 cable feeds ABS/EBS and the trailer's electrical systems.
- Spring brakes (fjederbremse) are held off by air pressure. Lose pressure and they apply - that is why a trailer with no red line is braked.
- EBS/ABS warning lamps must go out after the self-test; a lamp that stays on is a defect, not a nuisance.
The coupling routine - in order
- Check the fifth wheel is open, the kingpin and jaw are undamaged, and the trailer parking brake is applied.
- Reverse under squarely; listen and feel for the lock, then check the locking indicator and close the safety catch.
- Connect the red line first, then yellow, then the electrical cable; raise the landing legs fully.
- Do the pull test (trækprøve): apply the tractor's parking brake, select a low gear and try to creep forward against the trailer brake. The combination must not separate.
- Test the trailer service brake and check for leaks before moving off.
Worked example - no pressure build-up. After coupling, the gauge stays below 6 bar and the low-pressure warning sounds. Reasoning: the trailer is not receiving supply air. Check that the red coupling head is fully engaged and the seal is intact, that the yellow and red heads are not swapped, then drain any water from the reservoirs and look for a leaking hose. Do not drive; with insufficient pressure the trailer brakes may stay applied or release unpredictably.
Traps that cost marks
- Connecting yellow before red, so the trailer never builds supply pressure.
- Skipping the pull test because the locking indicator looks right.
- Forgetting the trailer parking brake before coupling or uncoupling, or failing to chock the wheels.
- Ignoring an ABS/EBS warning lamp or a slow pressure drop overnight.