What is a key component of trailer brake systems?

Trailer brake systems primarily rely on hydraulic fluid as a means of transmitting force to engage the brakes. When the brake pedal is pressed, the hydraulic fluid is forced through the brake lines, resulting in the activation of the brake components on the trailer. This hydraulic action is crucial because it provides the necessary force to stop the trailer effectively and safely.

Understanding the function of hydraulic fluid in trailer brakes also highlights the importance of maintaining proper fluid levels and ensuring that the hydraulic system is free of leaks. This ensures optimal braking performance and safety while towing. In contrast to the other options, which do not serve this specific function, hydraulic fluid plays a direct and vital role in the operation of the brake system.