CDL Vehicle Combination Practice Test

After pushing in the trailer supply valve, when should you move the tractor?

1. When the air system is at normal pressure

2. Immediately

3. After honking the horn

4. After checking the trailer load

The correct answer is: When the air system is at normal pressure

Moving the tractor after pushing in the trailer supply valve should only occur when the air system is at normal pressure. This is important because the trailer's braking system relies on a sufficient amount of air pressure to operate effectively. When the trailer supply valve is pushed in, it connects the tractor's air system to the trailer, allowing air to fill the trailer's brake tanks. If the air pressure is not at the correct level, the trailer brakes may not disengage properly or could fail to operate as expected, leading to safety hazards while on the road. Proper operational procedures dictate that the air pressure should reach at least 90-100 psi before the vehicle is set in motion. This ensures that both the tractor and trailer are ready to operate under safe conditions. Moving the tractor when the air system is not fully pressurized could compromise control and stopping capability, increasing the risk of accidents. The other options may suggest taking action that could lead to unsafe scenarios due to insufficient air pressure or checks not being performed.