CDL Vehicle Combination Practice Test

If the service air line comes apart while driving a combination vehicle but the emergency line stays intact, what is the immediate effect?

1. Brakes will engage automatically

2. Nothing until you attempt to apply the brakes

3. Vehicle will stop suddenly

4. Warning alarms will activate

The correct answer is: Nothing until you attempt to apply the brakes

When the service air line separates while driving a combination vehicle, but the emergency line remains connected, the immediate effect is that there will be no response until the driver attempts to apply the brakes. This is because the emergency line is responsible for maintaining pressure and activating the emergency brake system in case of a serious malfunction, while the service air line is used for regular braking operations. Since the service line has become disconnected, the air pressure needed to engage the service brakes will be absent. However, because the emergency line stays intact, the vehicle won't lose air pressure for emergency braking. Therefore, until the driver applies the brakes, there will be no reaction from the vehicle's braking system, as it relies on the service air line to function appropriately. Understanding this scenario is critical for safety in driving combination vehicles, as it underscores the importance of knowing how pneumatic braking systems operate and the implications of hardware failures.