A driver mistakenly connects the tractor's air-lines to the wrong glad hands, crossing the service and emergency lines, when coupling to a trailer. What is the practical danger of this mistake?

Explanation

Crossing the air-lines feeds supply air into the service line rather than charging the trailer's air tanks, so no air is available to release the trailer's spring brakes. On older trailers without spring brakes, the trailer's air tank never charges at all, meaning no emergency brakes and freely turning wheels, so the rig could be driven with no trailer brakes at all. This is exactly why the manual insists on testing trailer brakes before driving, and it is topic-scoped to 49 CFR 383.111(c)(3)(ii). Blaming the tractor's own system misattributes the danger to the wrong vehicle unit. Saying the mistake has no real effect is factually wrong; the two lines have genuinely different jobs, signal/control versus supply, so crossing them is not harmless. The ABS-lamp option confuses this air-line mistake with an unrelated electronic indicator.

Source: 49 CFR 383.111(c)(3)(ii)