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Illinois CDL Combination study materials
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Illinois CDL Combination practice questions

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Question 1Driving Combination Vehicles Safely

Before backing a tractor-trailer, the safest way to control the combination is to:

  1. Back only with the trailer's rear visible in the mirrors
  2. Get out and use hand signals while the engine idles
  3. Back only toward the driver's side with a spotter at the rear Correct answer.
  4. Back toward the right side whenever possible

Explanation

When backing a combination vehicle, you should back toward the driver's side so the trailer stays visible in the mirrors and you can correct steering more easily, and you should use a helper whenever one is available. Backing blind side or without a spotter greatly increases the risk of a jackknife or collision.

Question 2Driving Combination Vehicles Safely

The main purpose of a tractor protection valve on a combination vehicle is to:

  1. Keep the trailer brakes from releasing too quickly
  2. Balance air pressure between the tractor and trailer tanks
  3. Stop air loss from the tractor if the trailer breaks away Correct answer.
  4. Automatically apply the trailer spring brakes during normal driving

Explanation

The tractor protection valve closes the air lines to the trailer when pressure drops or the lines separate, preventing the tractor's air supply from being lost. This lets the tractor retain enough air to stop if the trailer breaks away.

Question 3Driving Combination Vehicles Safely

When you supply air to a trailer that has been dropped, the trailer's spring brakes will release only after:

  1. The glad hands are crossed
  2. The landing gear is fully raised
  3. The trailer air supply valve is pushed in and pressure builds Correct answer.
  4. The electrical cord is connected

Explanation

Spring brakes on a dropped trailer stay applied until air pressure builds through the emergency line. You must push in the trailer air supply valve and allow pressure to build before the brakes release.

Question 4Driving Combination Vehicles Safely

You are driving a loaded combination and must stop on a steep downgrade. The safest practice is to:

  1. Pump the brake pedal rapidly
  2. Hold the vehicle with the trailer hand valve only
  3. Rely on the engine brake alone
  4. Hold the vehicle with the service brake and set the parking brake Correct answer.

Explanation

On a grade, use the foot service brake to hold the vehicle and set the parking brake so the combination stays put. The trailer hand valve should not be used to hold a parked combination, and pumping brakes on a grade risks air loss.

Question 5Driving Combination Vehicles Safely

The term "crack-the-whip" effect refers to:

  1. The tendency of a lightly loaded trailer to bounce
  2. The trailer pushing the tractor during a downhill stop
  3. The tractor's rear wheels lifting under hard braking
  4. The rear trailer swinging out during a turn Correct answer.

Explanation

In a combination, the rear of the last trailer swings outward sharply during turns, and the effect grows toward the rear of the combination. This is why you must allow extra room for the trailer to clear curbs, poles, and other vehicles.

Question 6Driving Combination Vehicles Safely

When making a right turn in a combination vehicle, you should:

  1. Turn from the center lane without changing position
  2. Swing wide to the left before turning right Correct answer.
  3. Turn from the far left lane to widen the arc
  4. Keep the tractor close to the right curb and turn sharply

Explanation

Because the trailer cuts the corner, you should swing wide to the left before a right turn to keep the trailer's rear wheels from running over the curb or striking objects. Turning too tightly is a common cause of right-turn accidents.

Question 7Driving Combination Vehicles Safely

You are driving a combination vehicle and the trailer begins to jackknife during a hard stop. The best immediate action is to:

  1. Apply the trailer hand valve fully
  2. Release the brakes to let the trailer straighten Correct answer.
  3. Steer hard in the opposite direction
  4. Accelerate to pull the trailer straight

Explanation

Releasing the brakes allows the wheels to roll and the trailer to straighten behind the tractor, which helps recover from a jackknife. Steering or accelerating abruptly usually makes the situation worse.

Question 8Driving Combination Vehicles Safely

The required stopping distance for a combination vehicle compared with a single-unit truck of similar weight is:

  1. Shorter because of the extra axles
  2. Longer, especially on slippery surfaces Correct answer.
  3. Shorter because the trailer brakes assist
  4. About the same

Explanation

A combination vehicle generally needs more stopping distance than a single-unit vehicle, and the difference grows on slippery roads where the trailer can push the tractor. Following distance should be increased accordingly.

Question 9Driving Combination Vehicles Safely

When checking the coupling after connecting a tractor to a trailer, you should:

  1. Release the trailer brakes and roll forward
  2. Raise the landing gear before the tug test
  3. Pull gently forward with the trailer brakes set Correct answer.
  4. Connect the air lines after the tug test

Explanation

A tug test with the trailer brakes set confirms the kingpin is locked in the fifth wheel. If the trailer moves during the test, the coupling is not secure.

Question 10Driving Combination Vehicles Safely

The correct way to check that the trailer brakes are working after coupling is to:

  1. Check the trailer's electrical lights
  2. Listen for air escaping at the glad hands
  3. Pull against the trailer brakes with the tractor Correct answer.
  4. Watch the trailer wheels for movement

Explanation

With the trailer brakes applied, pulling gently against them should show resistance, confirming the trailer brakes are functioning. This is part of the pretrip and coupling inspection.

Question 11Driving Combination Vehicles Safely

When uncoupling a trailer, the first step after parking on level ground is to:

  1. Set the trailer brakes and chock the wheels Correct answer.
  2. Disconnect the air lines
  3. Release the fifth wheel
  4. Lower the landing gear

Explanation

Before uncoupling, set the trailer parking brakes and chock the wheels so the trailer cannot roll. Only then should you lower the landing gear and disconnect lines.

Question 12Driving Combination Vehicles Safely

The glad hands on a combination vehicle are used to:

  1. Secure the landing gear
  2. Connect the electrical wiring
  3. Lock the trailer to the fifth wheel
  4. Connect the air lines between tractor and trailer Correct answer.

Explanation

Glad hands are the couplers that join the tractor and trailer air lines. The service line is typically blue and the emergency line red, and they must be connected correctly for brakes to work.

Question 13Driving Combination Vehicles Safely

If the trailer's emergency air line separates from the tractor while driving, the trailer brakes will:

  1. Apply automatically Correct answer.
  2. Stay in the last position
  3. Apply only if the driver presses the pedal
  4. Release completely

Explanation

The emergency line loss causes the trailer spring brakes to apply automatically, stopping the trailer. This fail-safe design prevents a runaway trailer.

Question 14Driving Combination Vehicles Safely

When driving a combination vehicle through a curve, you should:

  1. Accelerate through the curve
  2. Brake hard in the curve
  3. Slow down before entering the curve Correct answer.
  4. Shift to neutral in the curve

Explanation

Speed should be reduced before entering a curve so the combination is stable through it. Braking or accelerating hard in a curve can cause the trailer to swing or roll.

Question 15Driving Combination Vehicles Safely

The safest way to drive a combination vehicle on a long downgrade is to:

  1. Use a gear lower than the one needed to climb the grade Correct answer.
  2. Ride the brakes lightly
  3. Use only the trailer hand valve
  4. Shift to neutral and coast

Explanation

Select a gear low enough that engine braking controls speed; the rule is to use a gear lower than the one used to climb the grade. This prevents brake fade from overheating.

Question 16Driving Combination Vehicles Safely

A combination vehicle is more likely to roll over than a single-unit vehicle mainly because:

  1. It has more tires
  2. It has a longer wheelbase
  3. Its center of gravity is higher and the trailer can swing Correct answer.
  4. It has a smaller turning radius

Explanation

Combination vehicles have a higher center of gravity and the trailer can swing outward, making them more prone to rollover, especially on ramps and curves. Load height and speed are major factors.

Question 17Driving Combination Vehicles Safely

When you must cross railroad tracks in a combination vehicle, you should:

  1. Shift gears while crossing
  2. Cross slowly in a low gear without stopping if possible Correct answer.
  3. Stop on the tracks if traffic is heavy
  4. Cross without stopping if the tracks are clear

Explanation

If traffic permits, cross the tracks without stopping and use a low gear to avoid shifting. Never stop on the tracks, and check clearance on the far side first.

Question 18Driving Combination Vehicles Safely

The purpose of a trailer's spring brakes is to:

  1. Reduce trailer sway
  2. Assist the tractor's service brakes
  3. Apply automatically when air pressure is lost Correct answer.
  4. Provide parking brake only

Explanation

Spring brakes are held off by air pressure and apply automatically when pressure drops, providing a fail-safe braking system. They also serve as parking brakes.

Question 19Driving Combination Vehicles Safely

You are driving a combination vehicle and the trailer starts to swing side to side. The best response is to:

  1. Ease off the accelerator and steer smoothly Correct answer.
  2. Accelerate to pull it straight
  3. Turn the steering wheel rapidly side to side
  4. Brake hard immediately

Explanation

Easing off the accelerator and steering smoothly helps settle trailer sway. Hard braking or rapid steering can worsen the sway and lead to a jackknife or rollover.

Question 20Driving Combination Vehicles Safely

The correct following distance for a combination vehicle traveling at highway speed should be:

  1. The same as a passenger car
  2. At least one second for each ten feet of vehicle length Correct answer.
  3. One second
  4. Two seconds regardless of speed

Explanation

A common rule is at least one second of following distance for each ten feet of vehicle length, and more in poor conditions. A typical combination needs about seven seconds at highway speed.

Question 21Driving Combination Vehicles Safely

When a combination vehicle's trailer is lightly loaded and the tractor is heavily loaded, the risk is that:

  1. The trailer brakes will fail
  2. The trailer will automatically lock
  3. The tractor will not start
  4. The trailer may skid or swing during hard braking Correct answer.

Explanation

A light trailer behind a heavy tractor can skid or swing during hard braking because the trailer has less weight to hold it down. Braking should be smooth and gradual.

Question 22Driving Combination Vehicles Safely

Before driving a combination vehicle, the driver should check that the fifth wheel is:

  1. Disconnected from the air lines
  2. Raised to its highest position
  3. Unlocked for easy turning
  4. Greased and locked around the kingpin Correct answer.

Explanation

The fifth wheel must be properly greased and locked around the kingpin so the trailer stays attached. An improperly locked fifth wheel can release the trailer while driving.

Question 23Driving Combination Vehicles Safely

When driving a combination vehicle, you should shift gears:

  1. Before entering a curve or downgrade Correct answer.
  2. While braking hard
  3. While crossing railroad tracks
  4. While turning at intersections

Explanation

Gears should be selected before curves and downgrades so the vehicle is in the proper gear and the driver's hands stay on the wheel. Shifting on tracks or in curves risks stalling or loss of control.

Question 24Driving Combination Vehicles Safely

When a combination vehicle's trailer has antilock brakes, the driver should:

  1. Pump the brakes to activate the system
  2. Keep steady pressure on the brake pedal Correct answer.
  3. Apply the trailer hand valve instead
  4. Release the brakes when the ABS light flickers

Explanation

With ABS, the driver should maintain steady brake pressure and let the system modulate the brakes. Pumping the pedal defeats the system and increases stopping distance.

Question 25Air Brakes and Brake System Checks on Combination Vehicles

On a combination vehicle with air brakes, what does the tractor protection valve do when air pressure drops to a low level?

  1. It releases the trailer supply line so the trailer can roll freely
  2. It closes and traps the tractor's remaining air while applying the trailer spring brakes Correct answer.
  3. It increases air flow to the trailer to help the trailer brakes apply faster
  4. It redirects air from the trailer tanks into the tractor tanks

Explanation

The tractor protection valve is a safety device that closes automatically when supply pressure falls too low, keeping the tractor's reserve air from escaping through a ruptured or disconnected trailer line. Closing the valve also causes the trailer emergency (spring) brakes to apply. This lets the driver retain enough pressure to stop the tractor safely.

Question 26Air Brakes and Brake System Checks on Combination Vehicles

During the pre-trip inspection of a combination vehicle, the trailer supply (emergency) line is pushed in and the trailer parking brakes should release. If the line is pulled out, what should happen?

  1. The trailer service brakes should apply gradually
  2. The trailer brakes should stay released until the foot valve is pressed
  3. The trailer spring brakes should apply Correct answer.
  4. Nothing should change until the engine is started

Explanation

Pulling the trailer supply valve knob (red) exhausts the trailer supply line, which triggers the trailer spring brakes to apply. This is the standard leak and function check for the trailer emergency system. The test confirms the trailer brakes will set if the lines separate.

Question 27Air Brakes and Brake System Checks on Combination Vehicles

You are doing a static leak test on a combination vehicle with a fully charged system. With the engine off and the brake released, pressure should not drop more than what amount in one minute?

  1. 3 psi Correct answer.
  2. 10 psi
  3. 20 psi
  4. 25 psi

Explanation

For a combination vehicle, the air loss rate with the engine off, the parking brakes released, and the service brakes not applied should not exceed 3 psi in one minute. This static leak standard is tighter than for a single vehicle because more lines and couplings are involved. A faster drop means a leak that must be found and fixed before driving.

Question 28Air Brakes and Brake System Checks on Combination Vehicles

With the engine off and the service brake fully applied, what is the maximum allowable air pressure loss in one minute for a combination vehicle?

  1. 2 psi
  2. 3 psi
  3. 1 psi
  4. 4 psi Correct answer.

Explanation

When the service brake is held fully applied with the engine off, the allowable loss for a combination vehicle is 4 psi in one minute. Holding the brakes applied opens more of the system to leakage, so the limit is slightly higher than the static rate. Exceeding it means the system needs repair before operation.

Question 29Air Brakes and Brake System Checks on Combination Vehicles

How do you perform the air compressor buildup check on a combination vehicle?

  1. Idle the engine and watch pressure fall from 100 to 85 psi
  2. Run the engine at fast idle and time how long pressure takes to rise from 85 to 100 psi Correct answer.
  3. Pump the brake pedal until the low-pressure warning activates
  4. Shut off the engine and listen for leaks at the glad hands

Explanation

The buildup test measures how quickly the compressor can raise system pressure. With the engine at fast idle, pressure should climb from 85 to 100 psi within about 45 seconds for a combination vehicle. A slower buildup points to a weak compressor, a slipping belt, or a leak.

Question 30Air Brakes and Brake System Checks on Combination Vehicles

At what pressure should the low air pressure warning device activate on a combination vehicle?

  1. When the trailer supply valve is pulled out
  2. Before pressure drops below 60 psi Correct answer.
  3. Only after pressure drops below 30 psi
  4. At exactly 100 psi

Explanation

The low air pressure warning, usually a buzzer and light, must come on before system pressure falls below 60 psi. This gives the driver time to stop safely before the spring brakes apply automatically. The warning is tested by pumping the brake pedal with the engine off.

Question 31Air Brakes and Brake System Checks on Combination Vehicles

You pump the brake pedal with the engine off to test the spring brakes. At what pressure should the trailer spring brakes begin to apply automatically?

  1. Between 80 and 100 psi
  2. Between 60 and 80 psi
  3. Between 20 and 45 psi Correct answer.
  4. At 120 psi

Explanation

As pressure falls into the 20 to 45 psi range, the spring brakes should start to apply because there is no longer enough air to hold the springs compressed. This is the automatic emergency feature of air-brake systems. The tractor protection valve also closes in this range.

Question 32Air Brakes and Brake System Checks on Combination Vehicles

After the spring brakes apply automatically during a low-pressure test, what should you do before the vehicle can move again?

  1. Release the parking brake knob only
  2. Rebuild air pressure until the spring brakes release Correct answer.
  3. Pump the foot valve repeatedly with the engine off
  4. Pull the trailer supply valve out and leave it out

Explanation

The spring brakes apply because air pressure is too low to hold them off. You must run the engine and let the compressor rebuild pressure above the release threshold before the brakes will release. Only then can the vehicle be moved safely.

Question 33Air Brakes and Brake System Checks on Combination Vehicles

What is the purpose of the trailer air supply valve (red knob) in the cab of a combination vehicle?

  1. It controls the trailer service brakes during normal stopping
  2. It adjusts the trailer relay valve sensitivity
  3. It controls air to the trailer emergency line and thus the trailer spring brakes Correct answer.
  4. It drains moisture from the trailer air tanks

Explanation

The red trailer air supply valve controls air pressure in the emergency (supply) line. Pushing it in charges the trailer system and releases the trailer spring brakes; pulling it out exhausts the line and sets them. It is the driver's main control over the trailer emergency brake system.

Question 34Air Brakes and Brake System Checks on Combination Vehicles

Which color identifies the trailer service (control) line glad hand on a combination vehicle?

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

Explanation

The service or control line, which carries the signal to apply the trailer service brakes, uses blue glad hands. The emergency or supply line uses red. Matching colors helps prevent cross-connection during hookup.

Question 35Air Brakes and Brake System Checks on Combination Vehicles

You are coupling to a trailer and notice the trailer air lines are crossed, with red connected to blue. What is the correct action?

  1. Charge the system anyway and rely on the foot valve
  2. Cap one line and drive with trailer service brakes only
  3. Swap the lines so colors match before charging the system Correct answer.
  4. Leave them crossed but pull the supply valve out while driving

Explanation

Crossed lines prevent the trailer brakes from working correctly and can cause the spring brakes to stay applied or the service brakes to malfunction. The lines must be reconnected color to color, red to red and blue to blue, before the system is charged. Never drive with crossed lines.

Question 36Air Brakes and Brake System Checks on Combination Vehicles

During a pre-trip inspection, how do you test that the trailer service brakes actually apply from the cab?

  1. Pull the red supply valve and listen for the pop
  2. Charge the system, release the parking brake, then press the foot valve and have someone watch the trailer brake chambers Correct answer.
  3. Rev the engine and watch the pressure gauge rise
  4. Disconnect the blue line and check for air escape

Explanation

To confirm the trailer service brakes work, the system must be charged and the parking brake released so the foot valve can send air through the blue line. Pressing the brake pedal should move the trailer brake pushrods. A helper watching the chambers confirms the application.

Question 37Air Brakes and Brake System Checks on Combination Vehicles

What does the trailer hand valve (trolley valve) do on a combination vehicle?

  1. It dumps air from the trailer supply line
  2. It releases the trailer spring brakes after a low-pressure event
  3. It applies the tractor spring brakes for parking
  4. It applies only the trailer service brakes for testing Correct answer.

Explanation

The trailer hand valve, often called the trolley valve, sends air to the trailer service brakes only. It is useful for checking that the trailer brakes apply while the vehicle is stationary. It should not be used for normal stopping because it does not apply the tractor brakes.

Question 38Air Brakes and Brake System Checks on Combination Vehicles

Why should you avoid using the trailer hand valve for routine stopping of a combination vehicle?

  1. It applies the tractor brakes too hard and overheats them
  2. It applies only the trailer brakes, which can cause a jackknife or skid Correct answer.
  3. It disables the low pressure warning
  4. It automatically sets the spring brakes

Explanation

The hand valve applies only the trailer service brakes, so the trailer can slow while the tractor keeps rolling, creating a jackknife risk especially on slick surfaces. Normal stops should use the foot valve, which applies both tractor and trailer brakes together. The hand valve is intended for testing.

Question 39Air Brakes and Brake System Checks on Combination Vehicles

You are parked on a grade with a combination vehicle. Which brake application keeps the vehicle held most reliably?

  1. Use the trailer hand valve only
  2. Leave the transmission in gear without brakes
  3. Set the parking brake control so the tractor and trailer spring brakes apply Correct answer.
  4. Hold the foot valve down with the engine running

Explanation

The parking brake control exhausts air so the spring brakes on both tractor and trailer apply and stay applied without driver effort. Holding the foot valve only works while pressure remains and the driver keeps pressing. Spring brakes are the designed holding brake for parking.

Question 40Air Brakes and Brake System Checks on Combination Vehicles

What is the correct order for performing the air brake checks during a pre-trip inspection of a combination vehicle?

  1. Check applied leak first, then static leak, then buildup
  2. Test the warning device first, then build pressure, then check leaks
  3. Build pressure, check static leak, check applied leak, test warning and spring brakes, then check buildup Correct answer.
  4. Check buildup first, then test spring brakes, then check leaks

Explanation

The logical sequence is to build full pressure, then check the static leak rate, then the applied-brake leak rate, then pump down to verify the warning and spring brake application, and finally check compressor buildup. This order uses the pressure you have built and avoids redundant work. Following a consistent sequence prevents skipped steps.

Question 41Air Brakes and Brake System Checks on Combination Vehicles

While driving, the low air pressure buzzer sounds and the gauge is falling. What is the safest immediate action?

  1. Move to a safe stop while you still have braking, then find the leak Correct answer.
  2. Pull the red supply valve out while moving at highway speed
  3. Shift to a lower gear and accelerate to build pressure
  4. Continue to the next stop and report it later

Explanation

A falling gauge with the warning sounding means pressure is approaching the point where spring brakes will apply automatically. The driver should stop safely while control remains, then investigate. Continuing to drive risks an automatic brake application in traffic.

Question 42Air Brakes and Brake System Checks on Combination Vehicles

What does the governor do in a combination vehicle air brake system?

  1. It controls when the compressor cuts in and out to maintain pressure Correct answer.
  2. It balances air between the tractor and trailer tanks
  3. It releases the spring brakes at a set pressure
  4. It limits the maximum trailer brake application pressure

Explanation

The governor senses system pressure and signals the compressor to load (cut in) when pressure drops and unload (cut out) when the upper limit is reached. This keeps pressure in the normal operating range. Typical settings are around 100 psi cut-in and 125 psi cut-out.

Question 43Air Brakes and Brake System Checks on Combination Vehicles

Your combination vehicle has a leaking trailer supply line coupling. What is the most likely symptom while driving?

  1. The governor cut-out pressure will drop to 60 psi
  2. Trailer spring brakes may apply and the low pressure warning may activate Correct answer.
  3. Trailer service brakes will apply harder than normal
  4. The compressor will stop running entirely

Explanation

A leak in the supply line reduces pressure available to hold off the trailer spring brakes, so they can drag or apply and the warning may sound. The system cannot keep up if the leak is large. The coupling should be repaired or replaced before driving.

Question 44Air Brakes and Brake System Checks on Combination Vehicles

When checking slack adjusters on a combination vehicle with automatic slack adjusters, what should you look for?

  1. Pushrod travel beyond the allowable limit with the brakes applied Correct answer.
  2. Grease fittings that are missing
  3. A gap between the chamber and the slack adjuster
  4. Slack adjusters that move freely with no pushrod travel

Explanation

Excessive pushrod travel with the brakes applied indicates the slack adjuster is not keeping the brakes in adjustment. Automatic slack adjusters should maintain travel within limits; if travel is excessive, the brake system needs service. Measuring travel is a required part of the pre-trip inspection.

Question 45Air Brakes and Brake System Checks on Combination Vehicles

On a combination vehicle, what is the purpose of the relay valve on the trailer?

  1. It regulates compressor cut-out pressure
  2. It sets the trailer spring brakes when parked
  3. It controls the tractor protection valve
  4. It speeds up application and release of the trailer brakes using air from the trailer tank Correct answer.

Explanation

The relay valve uses a small control signal from the blue line to open a larger passage from the trailer tank to the brake chambers. This makes trailer brake application and release faster and more responsive. Without it, the small control line would be slow to fill the chambers.

Question 46Air Brakes and Brake System Checks on Combination Vehicles

During the pre-trip inspection, you should check that the trailer air tank drain valves do what when opened?

  1. Release any accumulated moisture and then close tightly Correct answer.
  2. Discharge air only when the parking brake is set
  3. Stay open to keep the tank vented
  4. Release only when the engine is running

Explanation

Draining the trailer air tanks removes moisture that can freeze or corrode the system. Each drain should release any water, then close fully so the system holds pressure. A drain left open would cause a constant leak and prevent proper buildup.

Question 47Air Brakes and Brake System Checks on Combination Vehicles

You notice the air pressure gauge shows 90 psi with the engine running and the compressor cycling frequently. What does this suggest?

  1. The low pressure warning is faulty
  2. A leak or weak compressor is preventing normal pressure from being maintained Correct answer.
  3. The trailer spring brakes are released and holding pressure
  4. The governor is set correctly and the system is normal

Explanation

Normal operating pressure for a combination vehicle is roughly 100 to 125 psi. Pressure stuck near 90 psi with frequent compressor cycling suggests the system cannot keep up, often from a leak or a weak compressor. This should be diagnosed before the vehicle is driven.

Question 48Air Brakes and Brake System Checks on Combination Vehicles

What is the correct way to test the tractor protection valve during a pre-trip inspection?

  1. Pull the red knob and listen for air escaping from the blue line
  2. Charge the system, then pump the brakes down and observe that the valve closes near 20 to 45 psi, setting the trailer spring brakes Correct answer.
  3. Rev the engine and watch the gauge climb past 125 psi
  4. Disconnect the electrical cord and check the trailer lights

Explanation

To test the tractor protection valve, the system is charged and then pumped down with the engine off. As pressure falls into the 20 to 45 psi range, the valve should close and the trailer spring brakes should apply. This confirms the valve will protect the tractor if the trailer separates.

Question 49Air Brakes and Brake System Checks on Combination Vehicles

Why does a combination vehicle require a greater following distance and earlier braking than a single vehicle?

  1. It is heavier and its air brakes take time to apply through the longer system Correct answer.
  2. Its trailer lights are dimmer
  3. It has more mirrors and blind spots
  4. Its steering axle carries less weight

Explanation

A loaded combination is much heavier and its air brake system has to send signals and air through longer lines to the trailer, so there is a lag before full braking. The extra weight increases stopping distance. Earlier, smoother braking and more following distance compensate for both factors.

Question 50Air Brakes and Brake System Checks on Combination Vehicles

On a combination vehicle with a dual air brake system, which circuit supplies the trailer's service brakes and the tractor's rear brakes?

  1. The parking brake circuit
  2. The primary (rear) circuit Correct answer.
  3. The secondary (front) circuit
  4. The trailer supply circuit

Explanation

In a dual air brake system the primary circuit feeds the rear (drive) axle brakes and the trailer service brakes, while the secondary circuit feeds the front (steering) axle brakes. The two circuits are separated by a check valve so a failure in one does not drain the other. Knowing which circuit controls which brakes helps a driver interpret gauge behavior during a leak test.