Final answer:
The warning light for brake system failure due to low air will turn red when air pressure drops below the manufacturer's specified threshold, which must be checked in the vehicle's manual.
Step-by-step explanation:
The warning light for a brake system failure, relating to air pressure, will typically turn red if either the front or rear air pressure falls below a certain threshold. While the reference information mentions a pressure of 5.1 atm, this is not necessarily related to the specific pressure at which a brake system warning light will activate. Vehicle manufacturers will set the threshold for their air brake systems, and it's important to refer to the vehicle's manual to determine the exact pressure at which the warning light will be triggered. An air pressure that is too low in a braking system can be dangerous, as it may lead to brake failure.