Final answer:
In the event of a low air pressure warning for one of a dual air brake system, the vehicle should be stopped immediately and parked safely until the system is fixed.
Step-by-step explanation:
When your truck or bus has a dual air brake system and a low air pressure warning comes on for only one system, the correct course of action is to stop right away and safely park the vehicle. You should only continue driving after the faulty system has been properly fixed. Continuing to drive with a compromised air brake system can be dangerous as the braking efficiency is significantly reduced. It's crucial to ensure both systems are functioning properly to maintain safe braking capabilities.