Final answer:
The low air warning device on a CMVSS 121 compliant air brake equipped truck will activate at a minimum air pressure of 60 psi.
Step-by-step explanation:
The question relates to the air brake system on trucks that adhere to the Canadian Motor Vehicle Safety Standard (CMVSS) 121, which is a regulatory standard for air brake systems on heavy trucks and buses. Specifically, you're asking about the minimum air pressure level that triggers the low air warning device. For CMVSS 121 compliant vehicles, the low air warning device must activate at a minimum air pressure of typically 60 psi or greater. Drivers need to be aware of this to ensure they can take necessary safety precautions if the air pressure drops too low.