Massachusetts CDL: Combination Vehicles Practice Test

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 Control line and signal line

The service air line is also known as the control line or signal line.

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 A 5-axle tractor towing a 52-foot trailer

When any vehicle turns, its rear wheels will follow a shorter path than its front wheels. This is called off-tracking or "cheating." The longer the vehicle, the greater the difference. In a combination vehicle, the trailer's wheels will off-track more than the tractor's rear wheels. In a double or triple combination vehicle, the wheels of the last trailer will off-track the most.

Not quite. The answer is step on and off the brake pedal several times.

To test the tractor protection valve, charge the trailer air brake system, turn off the engine, and lower the air pressure by stepping on and off the brake pedal several times. The tractor protection valve control (also called the trailer air supply control) should pop out (or go from the “normal” to the “emergency” position) when the air pressure falls into the range specified by the manufacturer (typically 20–45 psi).

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 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 trailer emergency brakes to come on.

If air pressure in the emergency air line drops too low (say because of a bad leak in the service line), 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 To prevent steering problems

One of the first steps in coupling a tractor to a trailer is to check that the fifth wheel skid plate is greased as required. If the fifth wheel plate is not properly lubricated, friction between the plate and the trailer apron could cause steering problems.

Not quite. The answer is Bobtail tractors can take longer to stop than fully loaded combination vehicles.

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. Likewise, "bobtail" tractors (tractors without semitrailers) can take longer to stop than a fully loaded tractor-trailer. Be very careful when driving a "bobtail" tractor.

Not quite. The answer is The trailer emergency brakes will 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 directly in front of the trailer.

When you're ready to couple, position your tractor right in front of the trailer. If you were to back into the trailer at an angle, you might push it sideways and break the landing gear.

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 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 pushing in the trailer air supply control.

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.

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 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 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 the brakes on the trailer wheels caused the skid in the first place.

The trailer hand valve (also known as a trolley valve or Johnson bar) controls only the trailer brakes, not the tractor brakes. If you're already jackknifing because the trailer wheels locked up, applying the trailer brakes with the hand valve can make it even worse. Instead, stop braking. The rear wheels will start to roll again and you can steer to correct the skid.