Trucking cargo damage is not directly related to the current health of the truck cab or trailer.
When the mission starts, each type of cargo (normal/fragile/hazardous) is given an inital health value (500/100/50 respectively).
Once a second the health of the truck cab and the trailer is checked, and if it reduced since the last check, the amount of health lost is taken away from the cargo's health value. If the cargo health reaches 0 the mission fails.
It should also be noted the truck cab and trailer health must be 75%+ to start trucking, but after that the health doesn't matter unless it's so bad the engine is broken.
I don't know what problem you're trying to describe but I don't think there's an issue here, it's just a misunderstanding in terms of how it's supposed to work.