A combination vehicle's trailer brakes are badly out of adjustment while the tractor brakes are properly adjusted. Why is this a safety concern on a long downgrade?

49 CFR 383.111(b)(7)(i) – Proper braking techniques; AAMVA CDL Manual, Section 5, subsection 5.4.5 (p. 5-9)

A single out-of-adjustment brake does not just reduce its own stopping power, it quietly overloads every other brake on the vehicle. That is why the manual stresses checking adjustment often, especially when brakes are already hot from a grade, rather than assuming one weak brake is a minor problem.