Combination Vehicle CDL Practice Test

1. In a trailer with no spring brakes, the __________ brakes will take over when the air stored in the trailer air tank is gone.

• A. service
• B. parking
• C. emergency
• D. none of the above

Explanation:
Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes which work from the air stored in the trailer air tank. The emergency brakes come on whenever air pressure in the emergency line is lost. These trailers have no parking brake. The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected. But the brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away and then there will be no brakes. Therefore, it is very important for safety that you use wheel chocks when you park trailers without spring brakes. A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on. You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

In the handbook:
6.2.: Combination Vehicle Air Brakes - Trailer Service, Parking and Emergency Brakes

2. Supply air to the service line during the pre-trip inspection with the __________

• A. trailer air supply knob.
• B. trailer hand brake.
• C. emergency line shut-off valve.
• D. none of the above

Explanation:
To check that air flows to all trailers during your pre-trip inspection, use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for the air pressure to reach normal, then push in the red "trailer air supply" knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), and then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly(ies) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

In the handbook:
6.5.: Inspecting a Combination Vehicle - Combination Vehicle Brake Check

3. When uncoupling, pull the tractor partially clear of the trailer __________

• A. before you secure the tractor.
• B. before you unlock the fifth wheel.
• C. after you inspect the trailer supports.
• D. both A and C

Explanation:
Knowing how to couple and uncouple correctly is basic to safe operation of combination vehicles. Coupling and uncoupling incorrectly can be very dangerous. General coupling and uncoupling steps can be helpful, but there are differences between different rigs. Learn the details of coupling and uncoupling the truck(s) you will operate. To uncouple safely, unlock the fifth wheel in step six. Raise the release handle lock. Pull the release handle to the "open" position. Keep your legs and feet clear of the rear tractor wheels to avoid serious injury in case the vehicle moves. Step seven is to pull the tractor partially clear of the trailer. Pull the tractor forward until the fifth wheel comes out from under the trailer. Stop with the landing gear under the trailer (this prevents the trailer from falling to the ground if the landing gear should collapse or sink). In step eight, secure the tractor by applying the parking brake and placing the transmission in neutral. Step nine is to inspect the trailer supports. Make sure the ground is supporting the trailer and that the landing gear is not damaged.

In the handbook:
6.4.: Coupling and Uncoupling - Uncoupling Tractor-Semitrailers

4. The service line carries air which is controlled by the __________

• A. trailer hand valve.
• B. trailer hand brake.
• C. tractor protection valve.
• D. none of the above

Explanation:
Every combination vehicle has two air lines, the service line and the emergency line. They run between each vehicle (tractor to trailer, trailer to dolly, dolly to second trailer, etc.). The service line (also called the control line or signal line) carries air which is controlled by the foot brake or the trailer hand brake. Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change. The service line is connected to relay valves. These valves allow the trailer brakes to be applied more quickly than would otherwise be possible.

In the handbook:
6.2.: Combination Vehicle Air Brakes - Trailer Air Lines

5. You should chock the trailer wheels during uncoupling if __________

• A. the trailer has spring brakes.
• B. the trailer has emergency brakes.
• C. the trailer has parking brakes.
• D. none of the above

Explanation:
Knowing how to couple and uncouple correctly is basic to safe operation of combination vehicles. Coupling and uncoupling incorrectly can be very dangerous. General coupling and uncoupling steps can be helpful, but there are differences between different rigs. Learn the details of coupling and uncoupling the truck(s) you will operate. To uncouple safely, step three is to chock the trailer wheels if the trailer doesn't have spring brakes or if you're not sure. (If the trailer doesn't have spring brakes, the air could leak out of the trailer air tank, releasing its emergency brakes. Without chocks, the trailer could move.)

In the handbook:
6.4.: Coupling and Uncoupling - Uncoupling Tractor-Semitrailers