Missouri CDL Practice Exam 2025 - Free CDL License Practice Questions and Study Guide

The trailer hand valve, also known as the "trailer brake" or "trolley valve," is designed to engage the trailer's brakes independently from the tractor's. While this feature can be beneficial in certain situations, using it while driving can lead to significant safety issues.

By engaging the trailer brakes suddenly or unnecessarily, you risk causing the trailer to skid. This is particularly dangerous when the road is slick or when the trailer is loaded unevenly. The independent activation of trailer brakes can disrupt the balance of the vehicle, increasing the chances of jackknifing or losing control. It's essential to maintain smooth and controlled braking to ensure that both the tractor and trailer remain stable.

Instead, maintaining control over braking with the primary vehicle system is crucial for safe driving. This approach helps ensure that braking is balanced between the tractor and trailer, thereby enhancing overall stability and control during the drive.