District of Columbia CDL: Combination Vehicles Practice Test

Not quite. The answer is the coupling is not right and should be fixed before you drive the coupled unit.

Don't drive the vehicle until you're sure that the coupling is correct. Check to make sure of the following: There is no space between the upper and lower fifth wheel, the fifth wheel jaws have closed around the shank of the kingpin, the locking lever is in the "lock" position, and the safety latch is in position over the locking lever. If anything isn't right, don't drive the vehicle until you've found and corrected the problem. If you drive it anyway, your trailer could come loose while you're driving.

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 the trailer brakes are locked.

During coupling, before backing under the trailer, set the trailer air supply control to shut off the air and lock the trailer brakes. On newer vehicles, this control is an eight-sided red knob. You pull it out to shut off the air.

Not quite. The answer is Fully raised with the crank handle secured in its bracket

Once you've coupled, raise the landing gear all the way up. If it's only partially raised, it could catch on something, such as the railroad tracks on a raised railroad crossing.

Not quite. The answer is makes firm contact with the ground and takes some weight off the tractor.

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 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.

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

With the loss of air pressure, the tractor protection valve will close and the trailer emergency brakes will come on.

Not quite. The answer is emergency line

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 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 empty.

Trucks brake best when they are used as they were designed: to carry a properly balanced load. When a combination vehicle is empty or lightly loaded, its braking distance increases. The very stiff suspension springs and strong brakes give poor traction and make it very easy to lock the wheels.

Not quite. The answer is Always

If there is a leak in the emergency air line, air will eventually leak out of the trailer air tank. If the trailer has air brakes but not spring brakes, the brakes will fail. Use chocks to prevent the trailer from rolling if its brakes fail.

Not quite. The answer is the tractor protection valve control (trailer air supply control) will pop out or go from "normal" to "emergency."

When you test the tractor protection valve, the tractor protection valve control (also called the trailer air supply control) should pop out or go from the "normal" to the "emergency" position. This shows that the tractor protection valve has closed automatically to keep the air pressure from falling too far.

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 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 It is used to supply the trailer with air, shut the air off, and apply the trailer emergency brakes.

On newer vehicles, the trailer air supply control is an eight-sided red knob on the dashboard of the tractor. The knob may be labeled "Pull to exhaust...Push to supply" or something similar. When you push the knob in, air will flow through the trailer's emergency air lines to charge the trailer air brake system. Pulling out the knob shuts off the flow of air and causes the trailer emergency brakes to come on.

Not quite. The answer is rollovers.

More than half of truck driver deaths in crashes are the result of truck rollovers. A full load increases the weight on the truck, making a jackknife less likely. But a full load also raises the truck's center of gravity, making a rollover more likely. Furthermore, many of America's expressways are old, with curved exit ramps that don't have the banking needed for today's combination vehicles.

Not quite. The answer is 90 degrees

When connecting the glad hands, press the two seals together with the couplers at a 90-degree angle to each other. A turn of the glad hand attached to the hose will join and lock the couplers.

Not quite. The answer is 4 psi per minute.

After the initial drop, the maximum acceptable leakage rates with the service brakes on are 3 psi per minute for single vehicles and 4 psi per minute for combination vehicles. If the leakage rate is higher, check for leaks and correct them.

Not quite. The answer is tilted down toward the rear of the tractor.

Before coupling, make sure that the fifth wheel is in the proper position. For one thing, it should be tilted down toward the rear of the tractor so that the trailer can easily slide into it.

Not quite. The answer is emergency

The tractor protection valve keeps air in the tractor if the trailer breaks away or develops a bad leak. If air pressure drops too low in the service line, the tractor protection valve will close. This stops air from escaping from the tractor and lets air out of the trailer emergency line, causing the trailer emergency brakes to come on.