Illinois CDL Practice Exam.

The blue hose couplers in a trailer's air system are specifically designed to supply air for the trailer's brakes. When connected, these couplers facilitate the flow of air from the towing vehicle to the trailer, allowing the trailer's braking system to function effectively. This is crucial for safe stopping and control of the trailer while it is in motion. The other actions mentioned, such as activating the emergency brake or engaging the clutch, do not directly involve the function of the blue hose couplers. Adjusting the trailer's height is unrelated to the air supply system, as it usually requires mechanical adjustments independent of the air braking system. Thus, the primary function of the blue hose is to ensure that the brakes can be operated properly by providing the necessary air pressure.