South Carolina CDL: Combination Vehicles Practice Test

Not quite. The answer is open

When uncoupling, unlock the fifth wheel by raising the release handle lock and pulling the release handle to the open position. When you do this, keep your legs and feet well clear of the tractor's rear wheels in case the vehicle moves.

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 (a) The tractor is on level ground.

When coupling a trailer to a tractor, it's a good idea to make sure that you're on a level surface free of dips or potholes. If you back the tractor over a large bump, dip, or pothole, you might damage something as you attempt to push the tractor underneath the trailer. On the other hand, the trailer itself need not be perfectly level. You want it to be aligned so that it will be raised slightly when the tractor is backed underneath it.

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

If a major leak develops in the service line, you may not notice anything wrong – until you try to apply the service brakes. 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.

Not quite. The answer is Never

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. But don't use it as a parking brake, especially if the trailer lacks spring brakes. If a leak causes loss of air pressure, the trailer brakes will fail. Don't use the trailer hand valve while you're driving either. Braking the trailer and not the tractor can cause the trailer to skid.

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 Drive normally and get it fixed later.

Losing the ABS function does not change the basic parts of your truck, so you will still have the same brakes that you have always had. Simply use them and get the ABS repaired as soon as possible. There is no need to panic, unless your system shows other and more serious warnings.

Not quite. The answer is Normal

To test the trailer air brake system, first charge it with air. Push in the red trailer air supply knob, or place the tractor protection valve control in the "normal" position. Wait until the air pressure reaches normal before you start your tests.

Not quite. The answer is go slowly around turns.

Because of rearward amplification, a trailer is more likely to roll over than a tractor. And if you're pulling doubles or triples, the rearmost (last) trailer is even more likely to roll over. To reduce the risk of trailer rollover, steer gently and smoothly, especially if you're pulling a double or triple. Slow down before entering a curve or turn.

Not quite. The answer is makes firm contact with the ground.

When uncoupling a loaded trailer, lower the landing gear until it makes firm contact with the ground. Then crank the landing gear in low gear for a few more turns – just enough to lift some weight off the tractor, but not so much that you lift the trailer off the fifth wheel. This will make it easier to unlatch the fifth wheel now and couple next time.

Not quite. The answer is over the locking lever.

The safety latch should be in position over the fifth wheel locking lever. On some vehicles, you may have to position it manually.

Not quite. The answer is the emergency and service air lines.

Before backing under the trailer, you must lock the trailer brakes – which means that you first have to make sure that the trailer brakes are working. Connect the emergency and service air lines and test the trailer brakes.

Not quite. The answer is No, because the brakes on the trailer wheels caused the skid to begin with.

The trailer skid was caused by the brakes locking the wheels. Applying the trailer hand brake might make that even worse. Instead, release the brakes so the trailer wheels can roll again and regain traction.

Not quite. The answer is the trailer's emergency brakes to come on.

The trailer air supply valve (which is manually controlled by the red dashboard knob) senses air pressure in the supply (emergency) line. If air pressure in the supply line is lost, the trailer air supply valve will close automatically, and the resulting loss of air pressure in the tractor protection valve will cause it to close as well. And loss of pressure in the supply line will cause the trailer emergency (spring) brakes to come on.

Not quite. The answer is there is enough clearance between the tractor frame and the landing gear to accommodate sharp turns.

After you've coupled the trailer and fully raised the landing gear, check that there is enough clearance between the rear of the tractor frame and the landing gear so they won't make contact even on sharp turns.

Not quite. The answer is release the brakes.

Once you realize your vehicle is in a skid, you should release the brakes so that the wheels can grip the road again. The trailer will start to follow the tractor and straighten out.

Not quite. The answer is the coupling surface of the trailer is just below the middle of the tractor fifth wheel.

The tractor fifth wheel should tilt downward toward the trailer. As you start to back the tractor under the trailer, the coupling surface of the trailer should be just below the middle of the tractor fifth wheel.

Not quite. The answer is open except for those on the last trailer.

The shut-off valves are located at the rear of each trailer to allow you to stop the air flow there. If there is another trailer behind this one, open the valves to allow air to flow to that trailer; otherwise, close the valves.

Not quite. The answer is spread evenly across the rig and as low as possible.

More than half of truck driver deaths in accidents are the result of truck rollovers. The higher the truck's center of gravity, the more likely it is to roll over. If the cargo is to one side, it will make the trailer lean, which can also make a rollover more likely. To help prevent a rollover, spread your cargo evenly and keep it as low as possible. Drive slowly through curves and turns.