Final answer:
The correct action to take when a low air pressure warning comes on for only one system in your truck or bus with a dual brake system is to stop immediately and investigate the issue.
Step-by-step explanation:
If a low air pressure warning comes on for only one system in your truck or bus with a dual brake system, the correct action to take is to stop immediately and investigate the issue. Continuing to drive or ignoring the warning can be extremely dangerous. Switching to the other brake system may not be possible if the warning is indicating a serious problem. Increasing the air pressure in the warning system is not the appropriate solution as it does not address the underlying issue.