A major leak develops in a trailer's service line. When will the driver typically notice a problem?

AAMVA CDL Manual, Section 6.2.8, p. 6-6

This explains why a pre-trip air brake check matters even when everything looks fine at a glance. A slow or moderate leak may not show symptoms until the driver is already on the road and applies the brakes, which is exactly when a surprise brake problem is most dangerous.