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Missouri CDL Combination study materials
Missouri CDL Combination study resources

Missouri CDL Combination practice questions

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Question 1Combination Vehicle Knowledge Test

Before backing a tractor-trailer, a driver should set up the vehicle so that the maneuver is made toward which side of the rig?

  1. The side with the curb
  2. The driver's side Correct answer.
  3. Whichever side has more room
  4. The passenger's side

Explanation

Backing toward the driver's side keeps the trailer visible in the left mirror and shortens the blind area behind the rig. Turning toward the passenger side hides the trailer's rear and makes corrections harder to judge.

Question 2Combination Vehicle Knowledge Test

When coupling a tractor to a semitrailer, a driver should check that the trailer kingpin is engaged by doing what?

  1. Pull the tractor forward gently while the trailer brakes are locked Correct answer.
  2. Check that the trailer lights come on
  3. Listen for the air dryer to cycle
  4. Look at the fifth-wheel jaws from the cab window

Explanation

A pull test with the trailer brakes set confirms the jaws have closed around the kingpin. If the coupling is incomplete, the trailer will not move with the tractor and the driver will see the gap at the fifth wheel.

Question 3Combination Vehicle Knowledge Test

The service air line between tractor and trailer is normally color-coded which way?

  1. Black
  2. Blue
  3. Green
  4. Red Correct answer.

Explanation

The service (primary) line is red and the emergency (supply) line is blue on most combination vehicles. Correct identification prevents crossed lines that can release the trailer brakes unexpectedly.

Question 4Combination Vehicle Knowledge Test

What is the purpose of the trailer hand valve (trolley valve) in a combination vehicle?

  1. To supply air to the trailer tires
  2. To park the trailer on a grade
  3. To test the trailer brakes only Correct answer.
  4. To release the tractor spring brakes

Explanation

The hand valve applies only the trailer brakes, which makes it useful for checking that the trailer brakes work. It should not be used for parking because it can leak down over time.

Question 5Combination Vehicle Knowledge Test

Why must a driver use a lower gear when driving a loaded combination vehicle down a long downgrade?

  1. To improve fuel mileage
  2. To keep the trailer brakes applied
  3. To keep the engine cool
  4. To use engine braking and avoid overheating the service brakes Correct answer.

Explanation

A lower gear lets engine compression hold the vehicle back so the service brakes are used less. Relying on the brakes alone on a long grade can cause them to fade or fail.

Question 6Combination Vehicle Knowledge Test

A combination vehicle with a light load tends to do what when the driver brakes hard?

  1. Steer more tightly than normal
  2. Lock the trailer wheels and jackknife more easily Correct answer.
  3. Stop in a shorter distance with no risk
  4. Use less air pressure than a loaded rig

Explanation

An empty or lightly loaded trailer has less weight on its tires, so the wheels lock easily under hard braking. Locked trailer wheels can swing the trailer out and cause a jackknife.

Question 7Combination Vehicle Knowledge Test

The shut-off valves at the rear of a trailer are used for what purpose?

  1. Draining the trailer's coolant
  2. Controlling air to the rear of a second trailer Correct answer.
  3. Bleeding moisture from the tanks
  4. Adjusting trailer brake slack

Explanation

The rear shut-off valves let a driver supply air to a second trailer in a double or triple configuration. They must be closed when no second trailer is attached so air is not lost.

Question 8Combination Vehicle Knowledge Test

Which condition most increases the chance of a trailer off-tracking problem on a tight turn?

  1. A short trailer turning at high speed
  2. A long wheelbase trailer turning at low speed Correct answer.
  3. A trailer with new tires
  4. A lightly loaded tractor

Explanation

Long trailers cut the corner and their rear wheels track inside the path of the tractor. The longer the trailer and the tighter the turn, the more the rear of the trailer swings toward the inside.

Question 9Combination Vehicle Knowledge Test

When inspecting a combination vehicle, where should a driver check for air leaks in the coupling area?

  1. At the battery box
  2. At the exhaust manifold
  3. At the steering gearbox
  4. At the glad hands and their seals Correct answer.

Explanation

The glad hands join the tractor and trailer air lines, and their rubber seals are a common leak point. A leak there can slowly drain air pressure and weaken the trailer brakes.

Question 10Combination Vehicle Knowledge Test

On a combination vehicle, the emergency (supply) air line is usually what color?

  1. Blue Correct answer.
  2. Red
  3. Yellow
  4. Brown

Explanation

The emergency line is blue and the service line is red. Knowing the color code helps a driver reconnect the lines correctly after uncoupling.

Question 11Combination Vehicle Knowledge Test

A driver is uncoupling a trailer on level ground. What should be done with the trailer spring brakes before pulling the tractor out?

  1. Set them so the trailer stays parked Correct answer.
  2. Leave them as they are after the air lines are disconnected
  3. Release them so the trailer can roll
  4. Apply them only after the tractor is clear

Explanation

Setting the trailer spring brakes keeps the trailer from rolling once the air lines are disconnected. The trailer brakes must be engaged before the tractor is pulled away.

Question 12Combination Vehicle Knowledge Test

Why should a driver avoid using the trailer hand valve to hold a parked combination vehicle on a slope?

  1. It prevents the tractor from starting
  2. It overrides the engine brake
  3. It applies only the trailer brakes and can leak off Correct answer.
  4. It wears out the trailer tires

Explanation

The hand valve applies only the trailer brakes, and air pressure can gradually escape so the brakes release. Parking requires spring brakes or wheel chocks to hold the rig.

Question 13Combination Vehicle Knowledge Test

What does the yellow ABS malfunction lamp on a combination vehicle indicate when it stays on?

  1. The ABS is fully operational
  2. The ABS has a fault and normal brakes still work Correct answer.
  3. The trailer brakes are locked on
  4. The air supply is overpressurized

Explanation

A lamp that stays on means the antilock system has detected a fault and is disabled. The regular service brakes still function, but the driver should have the system checked.

Question 14Combination Vehicle Knowledge Test

When making a lane change in a combination vehicle, a driver should allow extra time because of what factor?

  1. The trailer brakes apply automatically
  2. The steering is quicker than in a car
  3. The mirrors show a wider view
  4. The rig is longer and accelerates more slowly Correct answer.

Explanation

A combination vehicle is long and heavy, so it takes more distance and time to change lanes safely. The driver must also check that the trailer has cleared the lane before returning.

Question 15Combination Vehicle Knowledge Test

What is the correct order for connecting air lines when coupling a tractor to a trailer?

  1. Either order works because the lines are identical
  2. Connect the electrical cord before either air line
  3. Emergency line first, then service line Correct answer.
  4. Service line first, then emergency line

Explanation

Connecting the emergency line first charges the trailer system so the spring brakes release in a controlled way. The service line is connected after the emergency line is secure.

Question 16Combination Vehicle Knowledge Test

A driver's combination vehicle starts to skid with the trailer wheels locked. What is the best first action?

  1. Steer sharply toward the shoulder
  2. Pull the trailer hand valve
  3. Release the brakes to let the wheels roll Correct answer.
  4. Apply more pressure to the brake pedal

Explanation

Releasing the brakes lets the locked wheels roll again and regain traction so the driver can steer. Steering sharply or adding brake pressure makes the skid worse.

Question 17Combination Vehicle Knowledge Test

Why must a driver check the fifth-wheel jaws and the locking lever after coupling?

  1. To check the trailer tire pressure
  2. To verify the air tanks are full
  3. To confirm the kingpin is locked and the trailer cannot separate Correct answer.
  4. To make sure the trailer lights are working

Explanation

The jaws must be closed around the kingpin and the locking lever secured so the trailer cannot separate from the tractor. A visual check plus a pull test confirms the lock.

Question 18Combination Vehicle Knowledge Test

What is the safest way to control speed on a long, steep downgrade in a loaded combination vehicle?

  1. Shift to neutral and coast
  2. Select a low gear before the grade and use light, steady braking Correct answer.
  3. Ride the service brakes hard from the top
  4. Use the trailer hand valve continuously

Explanation

Choosing a low gear before the descent lets engine braking hold the rig back so the service brakes stay cool. Light, steady brake applications keep speed under control without overheating.

Question 19Combination Vehicle Knowledge Test

A driver must make a right turn with a long trailer. To avoid striking the curb with the trailer's rear wheels, the driver should do what?

  1. Turn from the far right lane and hug the curb
  2. Turn sharply and accelerate through the corner
  3. Swing wide to the left before the turn Correct answer.
  4. Use the trailer hand valve during the turn

Explanation

Off-tracking pulls the trailer's rear wheels inside the tractor's path, so the driver must swing wider before turning. Hugging the curb causes the trailer wheels to clip it.

Question 20Combination Vehicle Knowledge Test

Which of these is a sign that a combination vehicle's air system may have a leak?

  1. The air dryer purges after the compressor cycles
  2. The air pressure gauge drops steadily with the engine off and brakes released Correct answer.
  3. The trailer brakes release when the supply line is connected
  4. The low-air warning buzzer sounds once at startup

Explanation

A steady pressure drop with the system charged and brakes released points to a leak somewhere in the lines or fittings. The driver should find and repair the leak before driving.

Question 21Combination Vehicle Knowledge Test

A driver is backing a trailer that begins to drift toward the right in the mirrors. What correction keeps the maneuver controlled?

  1. Turn the steering wheel toward the left to correct the drift
  2. Accelerate to straighten the trailer
  3. Turn the steering wheel toward the right to follow the trailer Correct answer.
  4. Apply the trailer hand valve and stop

Explanation

When backing, the tractor steers the trailer, so turning the wheel toward the direction the trailer is drifting brings the trailer back in line. Overcorrecting in the opposite direction causes a jackknife.

Question 22Combination Vehicle Knowledge Test

Why does a combination vehicle need a longer stopping distance than a single-unit truck of similar weight?

  1. The rig has a shorter wheelbase
  2. The trailer's brakes may not apply at the same time as the tractor's Correct answer.
  3. The tractor's engine is smaller
  4. The trailer tires have less tread

Explanation

In a combination vehicle, the trailer brakes can lag behind the tractor brakes, so the trailer pushes the tractor and lengthens the stop. The driver must allow more following distance to compensate.

Question 23Combination Vehicle Knowledge Test

A driver is inspecting a double trailer setup and finds the rear shut-off valves open with no second trailer attached. What is the effect?

  1. Air escapes and the remaining trailer brakes may weaken Correct answer.
  2. The ABS lamp turns off
  3. The front trailer brakes apply harder
  4. The tractor's spring brakes release

Explanation

Open shut-off valves at the rear of the last trailer let supply air escape to the atmosphere. The loss of air pressure can weaken the trailer brakes and prevent full system charge.

Question 24Combination Vehicle Knowledge Test

What is the main reason a driver should walk around and check the coupling area after hitching a trailer?

  1. To check the trailer's cargo weight
  2. To verify the tractor's fuel level
  3. To confirm the jaws are closed, the lever is secured, and the air lines are not crossed Correct answer.
  4. To test the engine brake

Explanation

A walk-around confirms the fifth-wheel jaws are locked, the release lever is secured, and the service and emergency lines are connected to the right glad hands. This prevents separation and brake failure on the road.

Question 25Combination Vehicle Knowledge Test

A driver's rig has a trailer with spring brakes that will not release after coupling. What is the most likely cause?

  1. The trailer is overloaded
  2. The emergency line is not fully connected or the trailer air tank is empty Correct answer.
  3. The service line is connected backwards
  4. The ABS lamp is on

Explanation

Spring brakes release only when the trailer air system is charged through the emergency line. A loose glad hand or an empty trailer tank leaves the brakes applied until supply air builds.

Question 26Combination Vehicle Knowledge Test

In a combination vehicle, what does the trailer hand valve (trolley valve) operate?

  1. The tractor drive-axle brakes only
  2. The trailer spring brakes and parking brake
  3. The trailer service brakes only Correct answer.
  4. The tractor front steering-axle brakes only

Explanation

The trailer hand valve applies only the trailer service brakes, which is why it is used for a test of trailer brake function. It does not apply the tractor brakes or the trailer spring (parking) brakes.

Question 27Combination Vehicle Knowledge Test

What color is the emergency (supply) line gladhand on a combination vehicle?

  1. Black
  2. Green
  3. Red Correct answer.
  4. Blue

Explanation

The emergency or supply line is red and the service line is blue; the color coding helps a driver connect the lines correctly. Crossing them will leave the trailer without a proper air supply.

Question 28Combination Vehicle Knowledge Test

When coupling a tractor to a trailer, the tractor should be positioned so that the trailer kingpin will do what?

  1. Pass through the open jaws of the fifth wheel Correct answer.
  2. Enter the throat of the fifth wheel slightly left of center
  3. Rest on top of the locking jaws until the tractor is backed further
  4. Strike the center of the fifth-wheel plate

Explanation

The tractor is backed so the kingpin enters the open jaws of the fifth wheel and locks. The trailer should be at the correct height so the kingpin slides under the plate rather than striking it.

Question 29Combination Vehicle Knowledge Test

What is the purpose of the trailer spring brakes?

  1. They release the trailer brakes when the gladhands are disconnected
  2. They provide extra braking only on steep downgrades
  3. They apply automatically when air pressure is lost Correct answer.
  4. They hold the trailer only while the engine is running

Explanation

Spring brakes are held applied by heavy springs and are released by air pressure, so they set automatically when supply pressure drops. This is why a trailer will not roll away when its air is lost.

Question 30Combination Vehicle Knowledge Test

A driver is backing a tractor-trailer and the trailer appears to move toward the left in the mirrors. Which steering input corrects the drift?

  1. Turn the steering wheel toward the right Correct answer.
  2. Accelerate to pull the trailer straight
  3. Hold the wheel steady and continue backing
  4. Turn the steering wheel further to the left

Explanation

When backing, the bottom of the steering wheel moves the trailer in the direction you turn it. If the trailer drifts left, steer right to bring it back toward the desired path.

Question 31Combination Vehicle Knowledge Test

Which of these is a warning sign of an air leak in a combination vehicle's brake system?

  1. The low air pressure warning buzzer sounds shortly after the engine is shut off Correct answer.
  2. The air compressor cycles less often than usual
  3. The trailer brakes release faster than the tractor brakes
  4. The air pressure gauge rises above governor cut-out and stays there

Explanation

A system that loses pressure after shutdown, or that cannot hold pressure with the engine off, is leaking. The low-pressure warning coming on soon after shutdown is a clear symptom.

Question 32Combination Vehicle Knowledge Test

Why does a combination vehicle generally need more stopping distance than a single-unit truck of similar total weight?

  1. The rig has more weight and momentum spread over multiple units and takes longer to bring to a stop Correct answer.
  2. The added length increases the time needed to build air pressure
  3. The trailer brakes are always weaker than tractor brakes
  4. The fifth wheel reduces the tractor's brake efficiency

Explanation

A combination carries more total weight and momentum, and the trailer's brakes and tires must work with the tractor's. The result is a longer distance to stop than a comparable single unit.

Question 33Combination Vehicle Knowledge Test

When uncoupling on level ground, what should a driver do with the trailer spring brakes before pulling the tractor out?

  1. Set them so the trailer stays parked Correct answer.
  2. Leave them in whatever position they were in
  3. Release them so the trailer can roll freely
  4. Apply only the tractor parking brake and leave the trailer brakes released

Explanation

The trailer spring brakes must be set so the trailer cannot roll when the tractor is pulled away. The tractor parking brake should also be set during the process.

Question 34Combination Vehicle Knowledge Test

A driver is coupling to a trailer and the trailer's spring brakes will not release after the air lines are connected. What is the most likely cause?

  1. The tractor's foot valve is stuck open
  2. The trailer's ABS lamp is on
  3. The trailer supply valve is closed or the emergency line is not fully connected Correct answer.
  4. The trailer service line is connected to the wrong gladhand

Explanation

Spring brakes need supply air to release, so a closed trailer supply valve or an improperly seated emergency gladhand will keep them applied. Checking the supply valve and both gladhand seals is the first step.

Question 35Combination Vehicle Knowledge Test

What is the primary reason a driver should walk around and inspect the coupling area after hitching a trailer?

  1. To make sure the landing gear is still on the ground
  2. To check the trailer's tire pressure
  3. To verify the kingpin is locked in the jaws and the safety latch is secured Correct answer.
  4. To confirm the trailer lights are working

Explanation

A visual check confirms the kingpin is fully seated in the closed jaws and the locking lever is secured. A high hitch or partial lock can let the trailer separate from the tractor.

Question 36Combination Vehicle Knowledge Test

On a combination vehicle, what does a yellow ABS malfunction lamp that stays on indicate?

  1. The trailer's spring brakes are set
  2. The ABS is functioning normally and the lamp will clear itself
  3. The ABS has detected a fault and the vehicle may have reduced antilock capability Correct answer.
  4. The air compressor has failed

Explanation

A yellow ABS lamp that stays on after the self-test means the antilock system has a fault. Normal braking remains, but wheel lockup protection may be lost, so the driver should have it serviced.

Question 37Combination Vehicle Knowledge Test

A driver's combination vehicle begins to skid with the trailer wheels locked. What is the best first action?

  1. Apply the trailer hand valve to break the skid
  2. Increase brake pressure to stop faster
  3. Steer sharply in the direction of the skid
  4. Release the brake pedal to let the wheels roll again Correct answer.

Explanation

Releasing the brake pedal lets the locked wheels roll and regain traction so steering control returns. Once rolling, the driver can steer and reapply brakes smoothly.

Question 38Combination Vehicle Knowledge Test

A driver must make a right turn with a long trailer. What technique helps avoid striking the curb with the trailer's rear wheels?

  1. Approach close to the right curb and begin the turn early
  2. Accelerate through the turn to shorten the trailer's path
  3. Swing wide to the left before turning and watch the trailer's rear wheels in the right mirror Correct answer.
  4. Turn from the far left lane and swing wide across the lanes

Explanation

Off-tracking makes the trailer's rear wheels cut inside the tractor's path, so the driver swings wide before the turn and monitors the trailer wheels. Turning too tight or too early puts the trailer onto the curb.

Question 39Combination Vehicle Knowledge Test

What is the correct order for connecting the air lines when coupling a tractor to a trailer?

  1. Connect both lines at the same time to equalize pressure
  2. Connect the red emergency line first, then the blue service line Correct answer.
  3. Connect the red line only and leave the blue line capped
  4. Connect the blue service line first, then the red emergency line

Explanation

The emergency (red) line is connected first so the trailer receives supply air and the spring brakes release. The service (blue) line is connected second, and both should be checked for secure seating.

Question 40Combination Vehicle Knowledge Test

A driver is inspecting a double trailer setup and finds the rear shut-off valves open with no second trailer attached. Why is this a problem?

  1. The open valves prevent the first trailer's spring brakes from applying
  2. The open valves cause the ABS lamp to stay on
  3. The open valves have no effect until a second trailer is added
  4. The open valves will let the emergency line lose air and apply the spring brakes Correct answer.

Explanation

Open shut-off valves at the rear of a trailer allow supply air to escape, which can set the spring brakes and prevent proper brake operation. They must be closed when no second trailer is connected.

Question 41Combination Vehicle Knowledge Test

Why must a driver check the fifth-wheel jaws and locking lever after coupling?

  1. To verify the landing gear is raised
  2. To confirm the kingpin is locked and the trailer cannot separate from the tractor Correct answer.
  3. To make sure the trailer lights will work
  4. To check that the air lines are the correct color

Explanation

The jaws must close around the kingpin and the locking lever must be secured, or the trailer can separate while moving. A visual check and a pull test confirm a solid coupling.

Question 42Combination Vehicle Knowledge Test

A driver is inspecting a combination vehicle and needs to check for air leaks in the coupling area. Where should the driver look?

  1. At the tractor's front wheel seals
  2. At the fifth-wheel mounting bolts
  3. At the trailer's mud flaps
  4. At the gladhand connections and the trailer supply valve Correct answer.

Explanation

Gladhand seals and the trailer supply valve are the common leak points in the coupling area. Soapy water applied there will bubble if air is escaping.

Question 43Combination Vehicle Knowledge Test

A driver is coupling to a trailer and the kingpin rides over the top of the fifth-wheel plate. What is the correct response?

  1. Back up again with more speed to force the pin into the jaws
  2. Lower the landing gear and drive slowly to seat the pin
  3. Pull forward, raise the trailer to the correct height, and recouple Correct answer.
  4. Leave it as is and rely on the safety latch to hold the trailer

Explanation

A high hitch means the kingpin did not enter the jaws, so the trailer is not secured. The driver must pull out, adjust the trailer height so the kingpin passes under the plate and into the jaws, and recouple.

Question 44Combination Vehicle Knowledge Test

A driver with a loaded combination vehicle must stop quickly on a wet road. Why is the trailer more likely to swing out than with a light load?

  1. The heavier load increases the trailer's momentum, which can push the tractor during hard braking Correct answer.
  2. Wet roads reduce trailer tire traction only
  3. The trailer brakes apply harder when loaded
  4. The fifth wheel loses lubrication under load

Explanation

A heavy load gives the trailer more momentum, and if its brakes are not balanced with the tractor's the trailer can push the tractor and swing out. Smooth braking and proper brake adjustment reduce the risk.

Question 45Combination Vehicle Knowledge Test

A driver is preparing to back a long combination vehicle into a tight dock on the right side. Which setup gives the best control?

  1. Keep the tractor and trailer in a straight line and turn only after the trailer reaches the dock
  2. Position the rig so the trailer is angled toward the dock and back slowly with the trailer's right side visible in the mirrors Correct answer.
  3. Start the back from the left side of the lane so the trailer swings right
  4. Back quickly to keep the trailer from drifting

Explanation

Setting up with the trailer angled toward the dock and keeping its right side visible in the mirrors lets the driver monitor drift and correct early. Slow speed and small steering inputs maintain control.

Question 46Combination Vehicle Knowledge Test

A driver's combination vehicle has a trailer with spring brakes that will not release after coupling. What is the most likely cause?

  1. The trailer supply valve is closed or the emergency line is not fully connected Correct answer.
  2. The tractor's foot valve is stuck open
  3. The trailer's ABS lamp is on
  4. The trailer service line is connected to the wrong gladhand

Explanation

Spring brakes need supply air to release, so a closed trailer supply valve or a poorly seated emergency gladhand keeps them applied. Checking the supply valve and gladhand seals is the first step.

Question 47Combination Vehicle Knowledge Test

When a combination vehicle's air lines are properly connected, which gladhand supplies air for releasing the trailer spring brakes?

  1. The blue gladhand, which carries the service line
  2. The green gladhand, which carries the electrical signal
  3. The red gladhand, which carries the emergency line Correct answer.
  4. The black gladhand, which carries the tractor protection signal

Explanation

The emergency (supply) line is the red gladhand; it charges the trailer air tanks and provides the air pressure that releases the trailer spring brakes. The service line is blue and only applies the brakes when the foot valve is pressed. Without air on the red line, the trailer spring brakes stay applied.

Question 48Combination Vehicle Knowledge Test

A driver must make a tight right turn with a long combination vehicle. To keep the trailer's rear wheels from striking the curb, the driver should:

  1. Turn the steering wheel sharply as soon as the tractor's front bumper reaches the corner
  2. Begin the turn from the far left lane and swing wide before entering the intersection
  3. Accelerate hard through the turn so the trailer follows the tractor more closely
  4. Turn wide enough that the trailer's rear wheels clear the curb, watching the trailer in the right mirror Correct answer.

Explanation

Off-tracking means the rear wheels of a long trailer follow a shorter path than the tractor's front wheels, so the driver must swing wide and monitor the trailer's right side in the mirror. The goal is to let the trailer's rear wheels clear the curb without crowding other lanes. Sharp or early steering increases the chance of curb strike.

Question 49Combination Vehicle Knowledge Test

After coupling a tractor to a trailer, a driver performs a pull test. What is the correct procedure and what does a successful test confirm?

  1. With the trailer brakes released, back the tractor sharply to seat the kingpin; the fifth wheel should lock with an audible click
  2. With the trailer brakes set, pull forward gently against the kingpin; the tractor should stop without the trailer moving Correct answer.
  3. With the trailer brakes released, pull forward slowly and apply the hand valve; the trailer should stop the tractor and hold it
  4. With the trailer brakes set, drive forward at 10 mph and apply the foot brake; the trailer should skid straight

Explanation

The pull test is done with the trailer spring brakes applied; the driver tugs forward gently against the locked kingpin. If the coupling is secure, the tractor will stop and the trailer will not move. Releasing the trailer brakes defeats the test because the trailer can roll.

Question 50Combination Vehicle Knowledge Test

A driver is inspecting a double-trailer combination and finds the rear shut-off valves open with no second trailer attached. Why is this a problem?

  1. It disables the trailer ABS warning lamp on the dash
  2. It causes the tractor protection valve to supply air only to the front trailer
  3. It increases the service brake application pressure on the front trailer
  4. It allows air to escape from the emergency line, which can apply the spring brakes and prevent the combination from moving Correct answer.

Explanation

Open rear shut-off valves on a double-trailer setup let emergency-line air escape to the atmosphere. The loss of supply pressure causes the spring brakes to apply, and the tractor protection valve may pop out, immobilizing the combination. Shut-off valves must be closed when no trailer is connected.