Final answer:
The driver must be able to see a warning before air pressure in the service air tanks falls below a certain value, such as 60 psi, to ensure the proper functioning of the brakes and other pneumatic systems in the vehicle.
Step-by-step explanation:
The driver must be able to see a warning before air pressure in the service air tanks falls below a certain value, which is usually indicated in pounds per square inch (psi).
For example, a common value is around 60 psi. This means that if the air pressure in the service air tanks falls below 60 psi, a warning will be triggered to alert the driver.
It is important for the driver to be aware of low air pressure in the service air tanks to ensure the proper functioning of the brakes and other pneumatic systems in the vehicle.