Final answer:
The driver must be able to see a warning before air pressure in the service air tanks falls below D) 60 psi. This requirement ensures that the driver is alerted in time to safely stop the vehicle and prevent brake failure.
Step-by-step explanation:
The question asks what is the minimum air pressure level at which a driver must be able to receive a warning before air pressure falls too low in the service air tanks. To ensure safety and proper functioning of the air brake system in vehicles like commercial trucks, there is a requirement that a warning be given to the driver before the pressure in the service air tanks falls below a certain level. This is to prevent brake failure due to insufficient air pressure.
The correct answer to the question is: D) 60 psi. Federal regulations typically require that the warning device for commercial vehicles be activated when the air tank pressure falls below 60 psi (pounds per square inch). It is essential for drivers to be alerted at this threshold to allow sufficient time for the driver to safely bring the vehicle to a stop and avoid a potential accident.
This level is set to ensure that there is enough remaining air pressure to allow the driver to control the vehicle and bring it to a complete stop, should there be an issue with the air brake system that causes the air pressure to continue to drop. Maintaining proper air pressure is critical in commercial vehicle operation for both safety and vehicle performance.