Idaho CDL: Combination Vehicles Practice Test

Not quite. The answer is Gently pull forward in low gear against the locked trailer brakes; then look at it carefully.

After you've backed under the trailer and locked the kingpin into the fifth wheel, raise the landing gear slightly. Then with the trailer brakes still locked, gently pull the tractor forward to check that the trailer is locked onto the tractor.

Not quite. The answer is Apply and release the trailer service brakes while you listen for sounds.

If you cross the air lines, supply air will be sent to the service line instead of the supply line and the trailer air tanks. Without a supply of air, the trailer service brakes won't work. Therefore, to make sure that the air lines aren't crossed, send air to the trailer brakes by pushing in the trailer air supply control (red knob). Let the pressure normalize. Then turn off the engine and try the trailer brakes. You should see and hear the brakes in action and then see the air pressure normalize again.

Not quite. The answer is slightly raised.

After you've backed under the trailer and locked the kingpin into the fifth wheel, raise the landing gear slightly. Then with the trailer brakes still locked, gently pull the tractor forward to check that the trailer is locked onto the tractor.

Not quite. The answer is Turn wide as you complete your turn.

To turn right, you may need to turn wide to help prevent the rear end of your vehicle from overrunning the curb. However, if you swing wide to the left before you turn right, another driver may try to pass you on your right, which can lead to a collision. Instead, start the right turn with the rear of your vehicle as close to the curb as possible and then turn wide as you complete the turn. This will deny drivers room on the right to try to pass you.

Not quite. The answer is None

If there is space between the lower and upper fifth wheel after you have finished coupling, something is wrong. Perhaps the kingpin is on top of the closed fifth-wheel jaws, which could result in the trailer coming loose. Don't start to drive until you have identified and fixed the problem.

Not quite. The answer is red; blue

To keep the air lines from being mismatched, they can be labeled with metal tags or color coded. Blue is used for the service lines, and red is used for the emergency lines. Couple the air lines of the same color.

Not quite. The answer is It will keep dirt and water out of the lines.

Some vehicles are equipped with "dead-end" or dummy couplers. When the air lines are not connected to a trailer, connect the glad hands to the dummy couplers to keep water and dirt out of the glad hands and air lines. If your vehicle doesn't have dummy couplers, you may be able to connect the glad hands to each other.

Not quite. The answer is The rear trailer is twice as likely to turn over as the tractor.

Because of rearward amplification, the last trailer on a combination vehicle has the highest probability of rolling over. This is called the crack-the-whip effect.

Not quite. The answer is With the tractor frame under the trailer

After you've unlocked the fifth wheel, pull the tractor forward until the fifth wheel is out from under the trailer but the tractor frame is still under the trailer. This will prevent the trailer from falling onto the ground should the landing gear fail to support the trailer.

Not quite. The answer is carries air to a relay valve that allows the trailer brakes to be applied quickly.

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. The service line is connected to relay valves that allow the trailer brakes to be applied more quickly. The relay valves send air to the brake chambers in response to the air pressure in the service line.

Not quite. The answer is B.

If you swing wide to the left before you turn right, another vehicle may try to pass you on your right while you are turning. Instead, as shown in path B, make the right turn with the rear of your vehicle as close to the curb as possible and turn wide as you complete the turn. This will deny other vehicles enough room on the right to try to pass you.

Not quite. The answer is check the air line connections.

If you cross the emergency and service air lines, air will flow to the service line instead of the trailer air tanks. Air won't be available to release the spring brakes. Therefore, if the spring brakes don't release when you push the trailer air supply control, check the air line connections.

Not quite. The answer is When you lock the drive wheels on the tractor and the tractor spins out sideways as the trailer continues to push forward

A jackknife is usually the result of a drive-wheel skid. The tractor's rear drive wheels lock, and the trailer starts to push the tractor sideways.

Not quite. The answer is to test the trailer brakes.

The trailer hand valve (also known as a trolley valve or Johnson bar) controls only the trailer brakes, not the tractor brakes. You can use it to test the trailer brakes. Don't use it while you're driving, though. If you apply it while you're driving, you'll brake the trailers but not the tractor, which can cause a skid. Instead, use the foot brake, which sends air to the tractor brakes as well as the trailer brakes.

Not quite. The answer is secured but with enough slack for turns.

During your walk-around inspection, make sure that the electrical and air lines are secure but have enough slack to stay attached and secure even when the vehicle is turning.

Not quite. The answer is To prevent steering problems

There shouldn't be any space between the upper and lower fifth wheel. However, friction could cause steering problems. To reduce friction, make sure that the fifth wheel plate is well lubricated as required.

Not quite. The answer is The trailer emergency brakes will come on.

If the emergency (supply) air line breaks, the tractor protection valve will close to stop any more compressed air from leaving the tractor. Hence, the tractor's service brakes should still work. However, the loss of trailer air pressure from the ruptured line will cause the trailer emergency (spring) brakes to come on.

Not quite. The answer is 20 to 45 psi.

The tractor protection valve is located in the tractor, usually at the rear of the cab. Its purpose is to keep air in the tractor's air brake system should the trailer break away or develop a bad leak. The tractor protection valve is designed to close automatically if the air pressure falls into the range specified by the manufacturer, typically 20–45 psi.

Not quite. The answer is at normal pressure.

Before backing under a trailer during the coupling process, make sure that the air pressure is at normal levels so you'll be able to brake safely.

Not quite. The answer is applying and releasing the trailer brakes and listening for brake sounds.

After you've connected the air lines, double-check the connection and make sure the brakes are working. Shut off the engine so you can hear the brakes. Apply the brakes and listen for the sound of the brakes being applied. Release the brakes and listen for the hiss of escaping air.