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CDL Practice Test: Combination Vehicles

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CDL Practice Test: Combination Vehicles

Combination Vehicles Questions

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Good Luck!

What is the best way to test the Trailer Service Brakes?
  • With the engine off, step on and off the brake pedal several times to reduce the air pressure in the tanks and make sure the tractor protection valve pops out
  • Pull out the trailer air supply control and pull gently on the trailer with the tractor to make sure the brakes work
  • Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped
  • Release all brakes except the trailer service brakes using the trailer hand control valve (trolley valve) then walk back to the rear of the trailer and visually check to be sure the brakes are engaged
This is a question from page 54 - click here to look up the answer

Quote From Page 79 Of The Illinois CDL Manual:

The trailer service brakes: Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on. This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.)

TruckingTruth's Advice:

Many companies are no longer installing trailer hand control valves which is very unfortunate as this is really the only way to be certain the trailer service brakes are functioning properly. If your truck does not come equipped with a trailer hand control valve, test your brakes before you start driving on a major roadway. If you notice the truck struggling to stop, the problem most likely is with your trailer brakes and needs to be inspected.

Next
The trailer air supply control valve will pop out, thus closing the tractor protection valve, in what air pressure range?
  • 20 to 45 psi
  • 0 to 20 psi
  • 50 to 70 psi
  • 100 to 125 psi
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi. Tractor protection valve controls or "emergency" valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The "normal" position is used for pulling a trailer. The "emergency" position is used to shut the air off and put on the trailer emergency brakes.

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When does a trailer jackknife most often occur?
  • When the trailer is empty
  • When the trailer is fully loaded to maximum weight
  • When the trailer is loaded to half of its maximum weight
  • All trailers, regardless of weight or cargo placement, have an equal chance of jackknifing
This is a question from page 48 - click here to look up the answer

Quote From Page 71 Of The Illinois CDL Manual:

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a "trailer jackknife"

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What are Glad Hands?
  • Coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer
  • Caps which cover the valve stems on tires
  • Used to secure the trailer kingpin to the tractor 5th wheel
  • A name for the yellow and red air valves in the cab of a truck
This is a question from page 50 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

"Glad hands" are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer.

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What does the Service Air Line do?
  • Carries air controlled by the foot brake or the trailer hand brake
  • Controls the parking brakes
  • Controls the emergency brakes
  • Supplies air to the trailer air tanks
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The service line (also called the "control line" or "signal line") carries air controlled by the foot brake or the trailer hand brake. Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change. The service line is connected to relay valves. These valves allow the trailer brakes to be applied more quickly than would otherwise be possible.

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Which of the following statements about the Emergency Air Line is false?
  • It supplies air to the trailer air tanks
  • It carries air controlled by the foot brake or trailer hand brake
  • When the line loses air pressure, it causes the tractor protection valve to close
  • It controls the emergency brakes on combination vehicles
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The emergency line (also called the supply line) has two purposes:

  • (1) To supply air to the trailer air tanks
  • (2) to control the emergency brakes on combination vehicles.

Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The pressure loss could be caused by a trailer breaking loose, thus tearing apart the emergency air hose. Or it could be caused by a hose, metal tubing or other part that breaks, letting the air out. When the emergency line loses pressure, it also causes the tractor protection valve to close (the air supply knob will pop out).

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What is the Trailer Hand Valve?
  • The trailer release mechanism
  • A lever which only controls the trailer service brakes
  • A valve used to release the air lines from the tractor to the trailer
  • A lever which only controls the tractor service brakes
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The trailer hand valve (also called the "trolley valve" or "Johnson bar") works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle (including the trailer(s'). There is much less danger of causing a skid or jackknife when using just the foot brake.

Never use the hand valve for parking because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes.) Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.

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Next
The Tractor Protection Valve will close automatically in what air pressure range
  • 90 to 110 psi
  • 50 to 75 psi
  • 20 to 45 psi
  • 0 to 25 psi
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi).

TruckingTruth's Advice:

It is definitely worth memorizing the psi range the tractor protection valve will automatically close. Not only will you need to know this for the written and pre-trip exams, but you'll definitely want to know this info for real-world driving situations and when testing the air brake system.

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