What defines blended air in a dual air brake system?

Blended air in a dual air brake system refers to air that is mixed from both the primary and secondary circuits. In this braking system, the primary circuit is responsible for controlling the brakes on the drive axle(s), while the secondary circuit controls the brakes on the trailer axle(s). Blending air from both circuits is essential for ensuring that the braking system operates evenly and effectively, providing balanced stopping power across the entire vehicle. This helps prevent issues such as brake fade or uneven wear, enhancing the safety and performance of the braking system.

The other choices are not applicable. Air taken from the atmosphere for cooling does not pertain to braking functions, and air coming solely from the primary circuit would not allow for the needed balance and redundancy that dual circuit systems are designed to provide. Lastly, air filled with hydraulic fluid for braking does not describe a characteristic of air brake systems, which rely on compressed air rather than hydraulic fluid to operate their braking mechanisms.