Minnesota CDL: Combination Vehicles Practice Test

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 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 To engage the trailer brakes in case of air pressure loss

The emergency air line has two main functions: (1) It supplies air to the trailer air tanks to keep the trailer brake system charged. (2) It controls the vehicle's emergency brakes. If air pressure in the emergency air line drops too low (say because of a bad leak or damage due to the trailer breaking loose), the trailer emergency brakes will come on. Also, the tractor protection valve will close to keep air from escaping from the tractor.

Not quite. The answer is The rear wheels of the last trailer

Off-tracking is defined as the different paths taken by the front and rear wheels in a turn. The longer the vehicle, the greater the difference will be. Thus, the wheels on the front trailer will offtrack more than the tractor's rear wheels, and the wheels on the rearmost trailer will offtrack most of all.

Not quite. The answer is Nothing is likely to happen until you try to apply the brakes.

If a leak has developed in the service line, air tank pressure may remain close to normal – until you try to brake. Then the air loss from the leak will cause the air pressure to drop quickly. If the air pressure drops far enough, the emergency brakes will come on.

Not quite. The answer is your brakes might fail.

On a combination vehicle, the brakes on the tractor and all trailers must work properly. If your combination vehicle is equipped with air brakes, check that air flows to all trailers and test both the trailer service brakes and the trailer emergency brakes.

Not quite. The answer is keep the cargo as close to the ground as possible.

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.

Not quite. The answer is easier it is to turn over.

When you stack cargo high in a trailer, you raise its center of gravity, which makes the trailer more prone to rolling over. A fully loaded tractor-trailer is ten times more likely to roll over in a crash than an empty one.

Not quite. The answer is If you make a quick lane change or sudden movement of your steering wheel, your trailers will tend to swing out. The rearmost trailer will swing out the most and will be most likely to roll over.

A sudden maneuver by the tractor may cause the rearmost (last) trailer to swing violently and possibly roll over. This is known as the crack-the-whip effect. It is caused by rearward amplification: The farther a trailer is from the tractor, the more likely it is to start swinging dangerously out of control.

Not quite. The answer is If the trailer has no spring brakes, you will be able to drive away but you won't have trailer brakes.

Trailers built before 1975 are not required to have spring brakes. If you cross the air lines when coupling such a trailer, you may have no air pressure to supply the trailer brakes. You'll be able to drive, but you won't be able to brake the trailer. Always test the trailer air brakes before you start to drive.

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 supply air to the trailer system and then pull out the air supply knob.

Before you back under the trailer, you must lock the trailer brakes. First, supply air to the trailer air brake system and make sure that the system is working properly. Then set the trailer air supply control to lock the trailer brakes. To do this, pull out the red air supply knob (on newer vehicles) or move the tractor protection valve control to the emergency setting.

Not quite. The answer is Yes, if you're not sure of your path of travel.

Backing a combination vehicle is potentially hazardous. Before you start to back up, check your intended path of travel. Exit the cab and walk around the vehicle. Check your clearance to the sides and overhead, in and near the path of your vehicle.

Not quite. The answer is turn the crank a few more times to lift some weight off the tractor.

After the landing gear has made firm contact with the ground, turn the crank in low gear a few more times to take 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 and couple the trailer the next time.

Not quite. The answer is It should not be done.

The trailer hand valve (also known as a spike, a trolley valve, or a Johnson bar) controls only the trailer brakes, not the tractor brakes. If you apply it while you're driving, you'll brake the trailers but not the tractor. This can cause a trailer skid.

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 tractor protection valve to close.

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.

Not quite. The answer is Normal

To test the trailer air brake system, first charge it with air. To do this, push in the red trailer air supply knob, or (in older vehicles) move the tractor protection valve control from the "emergency" position to the "normal" position. Wait until the air pressure reaches normal before you check for crossed air lines.

Not quite. The answer is a fully loaded trailer

Trucks brake best when they are used as they were designed: to carry a properly balanced load. Empty trucks actually have longer stopping distances than loaded trucks because they have less traction. The stiff suspension springs and strong brakes give poor traction and make it easier to lock the wheels.

Not quite. The answer is Look under the vehicle for the ECU and wheel speed sensor wires coming from the back of the brakes.

You can tell if your vehicle has ABS by looking under the vehicle for the electronic control unit (ECU) and wheel speed sensor wires leading from the back of the brakes.