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Quizzes > General Knowledge & Mixed

CDL Combination Vehicles Practice Test

Quick, free combination vehicles practice quiz with instant results.

Editorial: Review CompletedCreated By: Teacherashley GiangrossoUpdated Aug 25, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration for CDL Combination Exam quiz on dark blue background

This quiz helps you prepare for the CDL combination vehicles test, covering coupling, uncoupling, doubles handling, and air leakage checks. Get instant feedback to spot weak areas before test day. For more focused practice, review your walk-around with the cdl pre trip practice test and sharpen hookups with the coupling and uncoupling quiz.

What is the maximum allowable air leakage rate for a combination vehicle when the engine is off and the brakes are applied?
2 psi per minute
5 psi per minute
4 psi per minute
3 psi per minute
Federal regulations specify that with the engine off and brakes applied, the acceptable leakage rate for combination vehicles is not more than 3 psi per minute. Exceeding this rate indicates a leak in the brake system that must be repaired before driving. Regular pre-trip inspections help catch excessive leaks early.
When coupling a tractor to a semi-trailer, how far should the tractor be positioned under the trailer during initial alignment?
At least one foot away from the trailer
Fully underneath the trailer
Slightly under the trailer
Slightly forward of the trailer
Positioning the tractor slightly under the trailer ensures the fifth wheel jaws can fully engage the kingpin. If the tractor is too far under or too far away, coupling may fail or damage can occur. Always perform a visual check before locking the fifth wheel.
What does the tractor protection valve on a combination vehicle protect?
Service brake lines
Tractor air supply lines
Fuel system
Trailer air supply
The tractor protection valve closes to protect the tractor's air brake system if trailer air supply pressure drops too low. This prevents loss of all air pressure from the tractor's reservoirs. It ensures that the tractor can still be braked safely.
What color is the service air line in a dual air brake system?
Green
Red
Blue
Yellow
In most air brake systems, the service line that controls trailer brakes is colored blue. The red line is the emergency or supply line that holds air for normal braking. Familiarity with these colors prevents misconnection during coupling.
During a pre-trip inspection, which component should be locked before driving with a semi-trailer attached?
Safety chains
Trailer landing gear
Fifth wheel locking jaws
Glad hands
The fifth wheel locking jaws must fully close around the trailer kingpin to prevent separation. A visual inspection confirms the jaws are engaged properly. If the jaws are not locked, the trailer can come loose.
What is the primary function of the glad hands on an air brake system?
Lock the fifth wheel
Support air tanks
Connect air lines between tractor and trailer
Secure electrical wiring
Glad hands are couplers that join the tractor's air lines to the trailer's air brake system. They ensure a tight seal to prevent air leaks. Proper connection of glad hands is critical for reliable brake operation.
Which action is required when uncoupling a semi-trailer from a tractor?
Chock the trailer wheels
Raise the landing gear as high as possible
Apply the tractor parking brake only
Disconnect electrical harness first
Chocking the wheels of the trailer prevents it from moving once the tractor pulls away. Wheels must be securely blocked before releasing the landing gear or air lines. This step is required to prevent rollaway incidents.
What type of brakes are typically used as parking brakes on combination vehicles?
Disc brakes
Spring brakes
Hydraulic brakes
Electric brakes
Spring brakes use powerful coil springs to hold the brakes on when air pressure is released. They serve as the parking and emergency brakes on most large combination vehicles. This design ensures brakes apply automatically if air pressure is lost.
What is the purpose of the glide-out pin on a sliding fifth wheel?
To lock the kingpin in place
To adjust air brake pressure
To secure fifth wheel position for different trailer lengths
To engage the parking brakes
The glide-out pin locks the sliding fifth wheel in position to match trailer kingpin locations. This allows drivers to adjust for different trailer lengths and weight distributions. Proper pin engagement prevents unintended sliding during transit.
When checking for air leaks in a double trailer combination, what should you listen for at the glad hands and converter dolly?
Clicking noise indicating lock engagement
Hissing sounds indicating a leak
Rattling sound from loose chains
No noise at all
A hissing sound at glad hands or the converter dolly indicates air is escaping from a loose or damaged connection. Identifying leaks early prevents brake failure or loss of trailer air supply. Always tighten or replace damaged fittings immediately.
At what minimum air pressure does the low-air warning signal activate in a combination air brake system?
100 psi
60 psi
40 psi
80 psi
Federal regulations require a warning signal to activate before air pressure drops below 60 psi. This alert gives drivers time to stop safely before spring brakes activate. Failure to heed the warning can lead to sudden brake engagement.
What is the proper procedure for conducting a tug test on a coupled tractor-trailer combination?
Release all brakes then pull forward forcefully
Use hand valve while moving slowly
Apply trailer brakes, release parking brakes, then pull forward gently
Apply tractor brakes only and pull backward
A tug test confirms the tractor and trailer are properly locked together. By applying trailer brakes and gently pulling forward, you remove slack and check that the fifth wheel holds. If the coupling fails, stop and re-couple before proceeding.
Under Federal regulations, how often must combination vehicle air brakes undergo a periodic inspection?
Every 24 months
Every 6 months
At least every 12 months
Every 3 months
A thorough inspection of air brakes is required at least once every 12 months under FMCSA rules. This ensures wear items and adjustment issues are identified early. Regular inspections reduce the risk of brake system failures.
When adjusting manual slack adjusters on a combination vehicle, what is the maximum allowable pushrod travel?
1 inch
2 inches
3 inches
0.5 inch
Pushrod travel exceeding 1 inch indicates an out-of-adjustment brake chamber. Proper adjustment ensures full brake application and avoids uneven braking. Inspect and adjust slack adjusters whenever travel exceeds limits.
Which valve in the air brake system controls air flow to the trailer brakes when you step on the brake pedal?
Foot valve
Relay valve
Quick-release valve
Tractor protection valve
The relay valve speeds up air delivery from the trailer air tanks to the brakes when you apply the foot valve. It reduces lag time, ensuring quicker brake response. Without it, trailer braking would be delayed.
How long should it take for air pressure to build from 50 to 90 psi in a properly functioning combination system?
60 seconds
45 seconds
30 seconds
90 seconds
Federal standards require air compressors to build from 50 to 90 psi within 45 seconds at idle. Slower build times indicate compressor or system leaks. Drivers must correct slow build-up before safe operation.
During an air leakage test with the engine off and brakes applied, air pressure drops from 120 to 116 psi in one minute. Is this acceptable?
No, because it exceeds the 3 psi per minute limit
Yes, because it is within the 6 psi per minute allowance
Yes, because the threshold is 5 psi per minute
No, because the engine must be running during the test
The maximum allowable leakage with engine off and brakes applied is 3 psi per minute for combination vehicles. A 4 psi drop exceeds that limit and indicates a leak. All leaks must be repaired before driving.
The converter dolly's hand-operated valve that allows the driver to apply trailer brakes independently is commonly called what?
Quick-release valve
Trolley valve
Relay valve
Control valve
The trolley valve, or hand valve, lets drivers apply trailer brakes manually without using the foot pedal. It is mainly used for testing and parking purposes. Use caution because it bypasses the tractor protection valve.
Which component reacts fastest when you step on the brake pedal in a combination air brake system?
Parking brakes
Tractor service brakes
Emergency brakes
Trailer service brakes
Tractor service brakes receive air directly from the foot valve, so they apply nearly instantly. Trailer brakes go through a relay valve, causing slight delay. Understanding this delay is critical for safe stopping distances.
What is the primary function of the wig-wag device in an air brake system?
Regulate air compressor output
Warn the driver of low air pressure
Control trailer hand brakes
Ensure even brake application
The wig-wag is a mechanical warning device that drops a flag into view if air pressure falls below safe limits. It alerts drivers before the spring brakes engage automatically. Proper functioning of the wig-wag is required by federal law.
According to FMCSA, how often must automatic slack adjusters be inspected?
Every 3 months
Every 24 months
Every 6 months
Every 12 months or 120,000 miles
FMCSA requires automatic slack adjusters be inspected at least every 12 months or 120,000 miles, whichever comes first. This ensures brake chambers maintain correct adjustment. Failing to inspect can lead to brake fade or uneven application.
What is the effect of having too much free play on a manual slack adjuster?
No noticeable effect
Increased stopping distance
Reduced pedal force
Faster brake response
Excessive free play means the pushrod must travel further before applying the brakes. This delay increases overall stopping distance. Regular adjustment keeps free play within safe limits.
What threshold will cause the tractor protection valve to close and protect the tractor air system?
Service line pressure exceeds 80 psi
Emergency line pressure drops below 60 psi
Tractor air supply exceeds 120 psi
Trailer air supply drops below 20 psi
The tractor protection valve is set to close if trailer air supply falls below about 20 psi. This prevents trailer leaks from depleting tractor reservoirs. Drivers must repair leaks to restore full system operation.
What can a damaged diaphragm in a relay valve cause in a combination vehicle?
Slower application of trailer brakes
Immediate spring brake application
No change in brake response
Loss of tractor braking
A torn or stiff diaphragm in the relay valve restricts air flow, delaying trailer brake application. This leads to uneven braking and increased stopping distances. Relay valves must be serviced or replaced if diaphragms fail.
What is the maximum allowable weight for a single steer axle on a combination vehicle under federal law?
12,000 lbs
10,000 lbs
14,000 lbs
16,000 lbs
Federal regulations limit the weight on a single steering axle to 12,000 pounds to protect road surfaces. Exceeding this can result in fines and increased pavement damage. Always check axle scales during inspections.
According to the Federal Bridge Formula, what is the maximum allowable weight for a tandem axle group spaced 50 feet apart?
80,000 lbs
70,000 lbs
55,000 lbs
34,000 lbs
The Federal Bridge Formula calculates allowable weight based on axle spacing. For a 50-foot tandem spacing, the formula yields about 55,000 lbs. This helps protect bridges and pavements from concentrated loads.
In a fully loaded combination vehicle during prolonged downhill driving, which brake system factor is least affected by rising temperature?
Service brake effectiveness
Brake fade
Brake drum temperature
Air reservoir pressure
While brake drums and linings heat up and lose friction, air reservoir pressure remains relatively stable. High temperatures affect brake components but not the compressed air in tanks. Monitoring air pressure ensures leaks and failures are detected.
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Study Outcomes

  1. Understand Air Leakage Rate for Combination Vehicles -

    Identify the maximum allowable air leakage rate for a combination vehicle to maintain safe braking performance, a critical requirement for the CDL combination exam.

  2. Apply Safe Doubles Handling Techniques -

    Demonstrate proper control methods and preventive measures when you turn suddenly while pulling doubles, ensuring vehicle stability and cargo safety.

  3. Recall Essential Pre-Trip Inspection Steps -

    Review and list key inspection points for cdl combination vehicles, enabling early detection of mechanical issues before highway operation.

  4. Analyze Combination Vehicle Test Answers -

    Dissect common question formats and answer choices to accurately select the correct responses on the combination vehicle portion of the cdl combination exam.

  5. Evaluate Risk Situations -

    Assess real-world driving scenarios involving combination vehicles to recognize hazards such as trailer sway and jackknife potential, and apply appropriate corrective actions.

Cheat Sheet

  1. Air Leakage Rate Standards -

    According to FMCSA Part 393 regulations, a combination vehicle must not exceed an air loss of 3 psi per minute with the engine off during a one-minute static hold. For example, if the system reads 90 psi, it should not drop below 87 psi after one minute - remember "Three per minute for combination."

  2. Coupling and Uncoupling Procedures -

    Follow the North American Standardized Coupling Procedure: inspect fifth wheel jaw engagement, ensure kingpin is locked, and verify release arm is secure. Use the mnemonic "Lock, Lube, Level" to confirm jaws are closed, plate is greased, and trailer is level before driving.

  3. Handling Doubles in Turns -

    When you turn suddenly while pulling doubles, trailers can "cut in" and increase off-tracking; start the turn wider and maintain a steady speed to keep both trailers aligned. University of Minnesota research recommends a "Lead-Lag-Lead" steering approach: lead with tractor, lag second trailer, then align on exit.

  4. Weight Distribution and Balance -

    Use calibrated scales to ensure no axle group exceeds its Gross Axle Weight Rating (GAWR) and that the Center of Gravity stays within manufacturer limits. A quick formula: GCWR ≥ (Truck Weight + Trailer Weight + Cargo Weight) to avoid overloading penalties and maintain stability.

  5. Brake System Inspection -

    Refer to the CVSA Brake Inspector's Manual for a comprehensive pre-trip: test trailer air leak-down rate, verify tractor protection valve activation, and confirm emergency brakes hold at 20 - 40 psi in the trailer tank. Always listen for air leaks near glad hands and inspect airlines for kinks or abrasions.

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