What is the primary purpose of low air pressure warning devices?

The primary purpose of low air pressure warning devices is to provide alerts when air pressure is critically low. This is crucial in combination vehicles because a low air pressure scenario can significantly impact the effectiveness of the vehicle's braking system, especially in vehicles equipped with air brakes.

When the air pressure drops below a certain threshold, the warning device activates, alerting the driver to this critical condition. If the air pressure is too low, it may lead to brake failure, making it vital for the driver to take immediate corrective action, such as safely pulling over and addressing the air pressure issue. This safety feature is essential in preventing accidents and ensuring safe vehicle operation.

Other options mention indicators for various situations, but they do not capture the specific and critical role of low air pressure warning devices in maintaining safe levels necessary for the vehicle's braking system to function properly.