CDL Vehicle Combination Practice Test

When should the hand valve be used with a combination vehicle?

1. Only during emergency stops

2. To assist in turning

3. To test the trailer brakes

4. Never

The correct answer is: Never

The hand valve, also known as the trailer brake control, should not be used during normal driving conditions, especially when operating a combination vehicle, as it is designed primarily for use in specific situations. One of the main purposes of the hand valve is to apply the trailer brakes independently of the service brakes of the tractor. This capability is important in specific scenarios like parking or during situations where you may want to control the trailer's speed independently from the tractor. Using the hand valve while driving can lead to skidding or loss of control because it could cause the trailer to brake while the tractor is still moving, potentially resulting in a jackknife situation. Thus, it should not be engaged while the vehicle is in motion under normal circumstances, which reinforces the choice of never using it while driving a combination vehicle. In certain cases, the hand valve can be employed when testing the trailer brakes during a pre-trip inspection or to bring the trailer to a stop in controlled circumstances. However, under standard operation, the statement aligns with safe driving practices and reflects the best practices for controlling a combination vehicle.