C D L C L A S S A E N D O R S E M E N T • E X A M P R E PA R AT I O N
Combination
Vehicles
Practice Test
Questions and Answers • 2026/2027 Edition
A complete 50-question practice examination for the Combination
Vehicles endorsement, aligned with FMCSA regulations and state
CDL manuals. Every question includes the correct answer and a
detailed rationale.
Commercial Driver's License Study Series
50 Questions • 5 Topic Sections • Passing Score 80%
ED I TI ON
,CDL Combination Vehicle Practice Test - Questions & Answers 2026/2027
COMMERCIAL DRIVER'S LICENSE | CLASS A ENDORSEMENT
CDL - Combination Vehicle Practice Test
Questions and Answers 2026/2027
50 80% 5 2
PRACTICE QUESTIONS PASSING SCORE - 40 OF 50 TOPIC SECTIONS POINTS PER QUESTION
How to Use This Practice Test
This practice set contains 50 multiple-choice questions divided into five sections of ten questions each,
mirroring the topic areas of the official state-administered Combination Vehicles knowledge test
required for a Class A Commercial Driver's License. Each question is followed by the correct answer,
marked [CORRECT] beside the right option, and a detailed rationale explaining why that answer is
correct and why the other choices are wrong. To simulate the real examination, cover the answer lines
with a sheet of paper, answer all ten questions in a section, and then check your work against the
marked options and rationales. Score two points per question; you must reach 40 correct answers out of
50, an 80 percent passing rate, to meet the standard used by most state licensing agencies. Review every
rationale carefully, including the ones you answered correctly, because the explanations reinforce the
FMCSA safety concepts that state exams draw from most heavily. When you can pass this set
comfortably, you are ready to schedule the official knowledge test at your state licensing office.
SECTION 1: AIR BRAKE SYSTEMS Questions 1-10
Q1. What is the function of the tractor protection valve?
A. To disconnect the fifth wheel during a turn.
B. To protect the tractor's air system if the trailer breaks away or develops a major leak.
[CORRECT]
C. To increase air pressure delivered to the trailer brakes.
D. To automatically activate the trailer's clearance lights.
Correct Answer: B
Rationale: The tractor protection valve's primary purpose is to safeguard the tractor's air supply. If a
major leak develops in the trailer's air system or the trailer separates from the tractor, this valve
closes automatically to prevent the loss of air from the tractor, ensuring the driver can still brake and
steer the power unit. Option A is wrong because the fifth wheel is released manually with its release
arm and locking mechanism, never by an air valve. Option C is wrong because valves do not create or
boost pressure; the compressor and governor regulate system pressure. Option D is wrong because
trailer lighting operates through the electrical connector, not the air lines.
CDL Study Series - Combination Vehicles 1
, CDL Combination Vehicle Practice Test - Questions & Answers 2026/2027
Q2. Glad hands are:
A. The locking jaws on the fifth wheel that grip the kingpin.
B. Coupling devices used to connect the service and emergency air lines from the tractor
to the trailer. [CORRECT]
C. The handles used to crank the trailer's landing gear up and down.
D. Emergency shut-off valves located on the trailer's air tanks.
Correct Answer: B
Rationale: Glad hands are the coupling devices that join the two air lines (service and emergency)
from the tractor to the trailer. They work like a handshake between a male and female connector and
are sealed with rubber gaskets to keep air from leaking; damaged or dirty seals are a common cause
of pressure loss. Option A describes the fifth wheel's locking jaws, a completely separate mechanical
connection. Option C describes the landing gear crank handle. Option D describes shut-off valves
(cut-out cocks), which are used to close off air lines on trailers not in use.
Q3. After connecting the glad hands to a trailer, what is the best way to test that the air lines have
not been crossed?
A. Charge the system, then apply and release the trailer brakes using the trailer hand
valve and brake pedal, and check that the trailer brakes apply and release properly.
[CORRECT]
B. Drive forward slowly at about 20 mph and make a full brake application to see how the trailer
responds.
C. Disconnect the emergency line and listen for air escaping from the trailer tanks.
D. Compare the positions of the metal tags attached to the two air lines.
Correct Answer: A
Rationale: The CDL manual procedure is to charge the trailer, apply and release the trailer brakes
with the hand valve and brake pedal, and confirm the brakes apply and release as they should. If the
service and emergency lines are crossed, supply air goes where it does not belong and the trailer
brakes will not operate correctly, which this test reveals before you move. Option B is wrong because
you should never discover crossed lines by road testing at speed. Option C only proves a line carries
air, not which function it serves. Option D is wrong because tags and hose colors help you match
hoses but do not prove the connections actually work.
Q4. What are the two purposes of the emergency (supply) air line?
A. To carry metered air from the brake pedal and to operate the trailer's turn signals.
B. To supply air to the trailer air tanks and to control the trailer's emergency brakes.
[CORRECT]
C. To connect the fifth wheel release arm and to lock the jaws around the kingpin.
D. To cool the air compressor and to drain moisture from the system.
Correct Answer: B
Rationale: The emergency line has two jobs: it sends compressed air from the tractor to fill the
trailer air tanks, and it carries the steady pressure that keeps the trailer's spring (emergency) brakes
released. When that pressure is lost, the spring brakes apply automatically, which is exactly what you
want in a breakaway. Option A wrongly describes metered brake-pedal air, which travels in the
CDL Study Series - Combination Vehicles 2
Combination
Vehicles
Practice Test
Questions and Answers • 2026/2027 Edition
A complete 50-question practice examination for the Combination
Vehicles endorsement, aligned with FMCSA regulations and state
CDL manuals. Every question includes the correct answer and a
detailed rationale.
Commercial Driver's License Study Series
50 Questions • 5 Topic Sections • Passing Score 80%
ED I TI ON
,CDL Combination Vehicle Practice Test - Questions & Answers 2026/2027
COMMERCIAL DRIVER'S LICENSE | CLASS A ENDORSEMENT
CDL - Combination Vehicle Practice Test
Questions and Answers 2026/2027
50 80% 5 2
PRACTICE QUESTIONS PASSING SCORE - 40 OF 50 TOPIC SECTIONS POINTS PER QUESTION
How to Use This Practice Test
This practice set contains 50 multiple-choice questions divided into five sections of ten questions each,
mirroring the topic areas of the official state-administered Combination Vehicles knowledge test
required for a Class A Commercial Driver's License. Each question is followed by the correct answer,
marked [CORRECT] beside the right option, and a detailed rationale explaining why that answer is
correct and why the other choices are wrong. To simulate the real examination, cover the answer lines
with a sheet of paper, answer all ten questions in a section, and then check your work against the
marked options and rationales. Score two points per question; you must reach 40 correct answers out of
50, an 80 percent passing rate, to meet the standard used by most state licensing agencies. Review every
rationale carefully, including the ones you answered correctly, because the explanations reinforce the
FMCSA safety concepts that state exams draw from most heavily. When you can pass this set
comfortably, you are ready to schedule the official knowledge test at your state licensing office.
SECTION 1: AIR BRAKE SYSTEMS Questions 1-10
Q1. What is the function of the tractor protection valve?
A. To disconnect the fifth wheel during a turn.
B. To protect the tractor's air system if the trailer breaks away or develops a major leak.
[CORRECT]
C. To increase air pressure delivered to the trailer brakes.
D. To automatically activate the trailer's clearance lights.
Correct Answer: B
Rationale: The tractor protection valve's primary purpose is to safeguard the tractor's air supply. If a
major leak develops in the trailer's air system or the trailer separates from the tractor, this valve
closes automatically to prevent the loss of air from the tractor, ensuring the driver can still brake and
steer the power unit. Option A is wrong because the fifth wheel is released manually with its release
arm and locking mechanism, never by an air valve. Option C is wrong because valves do not create or
boost pressure; the compressor and governor regulate system pressure. Option D is wrong because
trailer lighting operates through the electrical connector, not the air lines.
CDL Study Series - Combination Vehicles 1
, CDL Combination Vehicle Practice Test - Questions & Answers 2026/2027
Q2. Glad hands are:
A. The locking jaws on the fifth wheel that grip the kingpin.
B. Coupling devices used to connect the service and emergency air lines from the tractor
to the trailer. [CORRECT]
C. The handles used to crank the trailer's landing gear up and down.
D. Emergency shut-off valves located on the trailer's air tanks.
Correct Answer: B
Rationale: Glad hands are the coupling devices that join the two air lines (service and emergency)
from the tractor to the trailer. They work like a handshake between a male and female connector and
are sealed with rubber gaskets to keep air from leaking; damaged or dirty seals are a common cause
of pressure loss. Option A describes the fifth wheel's locking jaws, a completely separate mechanical
connection. Option C describes the landing gear crank handle. Option D describes shut-off valves
(cut-out cocks), which are used to close off air lines on trailers not in use.
Q3. After connecting the glad hands to a trailer, what is the best way to test that the air lines have
not been crossed?
A. Charge the system, then apply and release the trailer brakes using the trailer hand
valve and brake pedal, and check that the trailer brakes apply and release properly.
[CORRECT]
B. Drive forward slowly at about 20 mph and make a full brake application to see how the trailer
responds.
C. Disconnect the emergency line and listen for air escaping from the trailer tanks.
D. Compare the positions of the metal tags attached to the two air lines.
Correct Answer: A
Rationale: The CDL manual procedure is to charge the trailer, apply and release the trailer brakes
with the hand valve and brake pedal, and confirm the brakes apply and release as they should. If the
service and emergency lines are crossed, supply air goes where it does not belong and the trailer
brakes will not operate correctly, which this test reveals before you move. Option B is wrong because
you should never discover crossed lines by road testing at speed. Option C only proves a line carries
air, not which function it serves. Option D is wrong because tags and hose colors help you match
hoses but do not prove the connections actually work.
Q4. What are the two purposes of the emergency (supply) air line?
A. To carry metered air from the brake pedal and to operate the trailer's turn signals.
B. To supply air to the trailer air tanks and to control the trailer's emergency brakes.
[CORRECT]
C. To connect the fifth wheel release arm and to lock the jaws around the kingpin.
D. To cool the air compressor and to drain moisture from the system.
Correct Answer: B
Rationale: The emergency line has two jobs: it sends compressed air from the tractor to fill the
trailer air tanks, and it carries the steady pressure that keeps the trailer's spring (emergency) brakes
released. When that pressure is lost, the spring brakes apply automatically, which is exactly what you
want in a breakaway. Option A wrongly describes metered brake-pedal air, which travels in the
CDL Study Series - Combination Vehicles 2
