What is the primary function of brake lines in a trailer's braking system?

The primary function of brake lines in a trailer's braking system is to transport hydraulic fluid to the brake components. This is a critical role, as the braking system relies on hydraulic pressure generated by the master cylinder when the driver's foot presses the brake pedal. The brake fluid travels through the brake lines, transmitting force to the brake calipers or wheel cylinders, which then apply pressure to the brake pads or shoes against the brake rotors or drums. This action creates the friction needed to slow down or stop the trailer safely.

Understanding this function is key, as improper maintenance or damage to the brake lines can lead to brake failure or reduced braking efficiency, posing a safety risk. The other options relate to functions that are not relevant to the braking system, such as aerodynamics, electrical connections, or structural integrity—none of which pertain to the transport and application of brake fluid in the context of braking.