Study guide
13.4 Inspecting Combination Vehicles: Walk-Around Additions and the Combination Vehicle Air Brake Check
Commercial Driver's License CDL
Track your progress as you read
Finished sections get a check mark.
1 Chapter 1: CDL Overview, CLP, and Federal Licensing Regulations
1.1 CDL Classes, Endorsements, Restrictions, and Testing Architecture (49 CFR Part 383) 1.2 Entry-Level Driver Training (ELDT) and the Training Provider Registry (49 CFR Part 380) 1.3 Commercial Learner's Permit (CLP) Rules and Driver Disqualifications 1.4 Medical Self-Certification Categories, Other CDL Rules, and the International Registration Plan
2 Chapter 2: Vehicle Inspection and Maintenance
2.1 The Seven-Step Pre-Trip Inspection Method 2.2 Critical Safety Components: Steering, Suspension, Brakes, and Tires 2.3 En Route Inspections, Post-Trip DVIR, and Out-of-Service Criteria
3 Chapter 3: Basic Vehicle Control and Operating Fundamentals
3.1 Instrument Controls, Engine Starting, and Shutdown Procedures 3.2 Shifting Gears: Manual Transmissions, Double-Clutching, and Progressive Shifting 3.3 Backing Fundamentals, GOAL Principle, and Basic Maneuvering 3.4 Putting the Vehicle in Motion: Accelerating, Steering, and Stopping Smoothly
4 Chapter 4: Speed Management and Stopping Distances
4.1 The Three Components of Total Stopping Distance (Perception, Reaction, Braking) 4.2 Speed Management on Curves, Slippery Roads, and Downgrades 4.3 Hydroplaning, Speed vs. Weight, and Reduced Visibility Adjustments
5 Chapter 5: Space Management and Defensive Driving
5.1 Managing Following Distance and Space Ahead (The 1-Second per 10-Foot Rule) 5.2 Space Beside, Behind, and Overhead Clearance Management 5.3 Turning, Off-Tracking, and Managing Space in Intersections 5.4 Space Below and the Space Needed to Cross or Enter Traffic
6 Chapter 6: Visual Search, Hazard Perception, and Communication
6.1 Visual Search: Looking 12-15 Seconds Ahead and Mirror Monitoring (5-8 Seconds) 6.2 Identifying Hazards: Work Zones, Pedestrians, Distracted Drivers, and Slow Vehicles 6.3 Communicating Intent: Signaling, Flasher Use, Horn, and Breakdown Warning Triangles (49 CFR 392.22) 6.4 Distracted Driving and the Federal Mobile Phone and Texting Rules (49 CFR 392.80 and 392.82) 6.5 Aggressive Drivers and Road Rage: Recognition, Prevention, and Response
7 Chapter 7: Driving in Extreme and Adverse Conditions
7.1 Night Driving, Low-Beam/High-Beam Management, and Glare Recovery 7.2 Driving in Fog, Heavy Rain, Snow, and Severe Winter Conditions 7.3 Hot Weather Driving, Tire Blowout Prevention, and Radiator Overheating
8 Chapter 8: Emergency Maneuvers, Skid Control, and Accident Procedures
8.1 Evasive Steering, Off-Road Recovery, and Emergency Braking (Controlled vs. Stab) 8.2 Recognizing and Recovering from Drive-Wheel and Front-Wheel Skids 8.3 Post-Crash Procedures, Fires (Extinguisher Classes), and Emergency Incident Response
9 Chapter 9: Mountain Driving and Downgrade Techniques
9.1 Engine Braking, Auxiliary Retarders, and Gear Selection Before the Grade 9.2 Snub Braking Technique vs. Continuous Braking and Brake Fade 9.3 Runaway Truck Ramps and Emergency Descents
10 Chapter 10: Railroad-Highway Grade Crossings and Special Intersections
10.1 Railroad Crossing Procedures, Warning Systems, and Passive vs. Active Crossings 10.2 Mandatory Stop Requirements (Hazmat, Passenger, School Bus) and Clearance Safety 10.3 Low-Clearance and Low-Ground-Clearance Hanging Hazards at Crossings
11 Chapter 11: Cargo Securement and Weight Distribution
11.1 Federal Weight Limits (GVWR, GCWR, Axle Limits, Bridge Formula) and Center of Gravity 11.2 Cargo Securement Rules, Working Load Limits, Tiedowns, and Covering Requirements 11.3 Cargo Types: Liquids/Baffles, Dry Bulk, Over-Dimensional Loads, and Inspection Intervals (50 mi / 3 hr / 150 mi) 11.4 Blocking, Bracing, Header Boards, Covering Loads, and Sealed or Containerized Cargo
12 Chapter 12: Air Brake Systems Overview and Operation
12.1 Air Brake System Components: Compressor, Governor, Air Tanks, Safety Valves, and Drain Valves 12.2 Brake Chambers, S-Cam Mechanism, Slack Adjusters, and Spring Parking Brakes 12.3 Dual Air Brake Systems, Low Air Warning (60 psi), and Emergency Spring Brake Pop-Out (20-45 psi) 12.4 Air Brake Pre-Trip Inspection and Step-by-Step Leak Rate Testing 12.5 Antilock Braking Systems (ABS): Required Vehicles, Malfunction Lamps, and Braking With and Without ABS
13 Chapter 13: Combination Vehicles Overview and Coupling
13.1 Combination Vehicle Dynamics: Rollover Risk, Rearward Amplification, and Trailer Jackknifing 13.2 Coupling and Uncoupling Step-by-Step Procedures: Fifth Wheel, Kingpin, Air and Electrical Lines 13.3 Trailer Air Lines (Service vs. Emergency/Supply), Glad Hands, and Trailer Protection Valves 13.4 Inspecting Combination Vehicles: Walk-Around Additions and the Combination Vehicle Air Brake Check
14 Chapter 14: Driver Wellness, Hours of Service, and Drug/Alcohol Rules
14.1 Hours of Service (HOS) Regulations: 11-Hour, 14-Hour, 30-Minute Break, 60/70-Hour Rules & ELD Mandate 14.2 Driver Physical Qualifications, Medical Certification (Part 391), Fatigue, and Sleep Apnea 14.3 Drug and Alcohol Regulations (49 CFR Part 382): 0.04% BAC Limit, Testing Types, and Drug & Alcohol Clearinghouse
15 Chapter 15: Hazardous Materials Awareness for General CDL
15.1 Hazardous Materials Identification: The Nine Hazard Classes and UN/NA Numbers 15.2 Shipping Papers, Emergency Response Guidebook (ERG), and In-Cab Document Rules 15.3 Placarding Thresholds (Table 1 vs. Table 2, 1,001-lb rule), Bulk vs. Non-Bulk, and Hazmat General Rules
13.4 Inspecting Combination Vehicles: Walk-Around Additions and the Combination Vehicle Air Brake Check
Key Takeaways Explain This Topic AI
The seven-step inspection still applies to a combination vehicle; the coupling system, air and electrical lines, landing gear, and shut-off valves are additions to it, not replacements.
On the lower fifth wheel there must be no visible space between the upper and lower fifth wheel, and the locking jaws must be closed around the shank of the kingpin, not around the head.
All shut-off valves (cut-out cocks) in the service and supply lines must be OPEN except the ones at the back of the last trailer, which must be CLOSED.
Test the tractor protection valve by charging the system, shutting the engine off, and fanning the brakes down: the trailer air supply control should pop out in the manufacturer's range, usually 20 to 45 psi.
Test trailer service brakes with the hand valve to confirm the trailer brakes are connected and working, then control them with the foot pedal in normal operation.
Last updated: August 2026
Inspecting Combination Vehicles
Use the seven-step inspection procedure to inspect a combination vehicle. There are simply more things to inspect — more tires, wheels, lights, and reflectors — plus a set of items that exist only on a coupled vehicle. This section covers those additions and the combination-specific brake check. Both are separate testable areas, and the brake check in particular is a sequence you must be able to recite in order.
- 1. Coupling System Areas
- Lower Fifth Wheel
Securely mounted to the frame — inspect mounting bolts and welds.
No missing or damaged parts.
Enough grease on the plate. A dry fifth wheel binds in turns and can steer the tractor.
No visible space between the upper and lower fifth wheel. Daylight between the plates means the coupling is not fully seated — the single most dangerous finding in this whole inspection.
Locking jaws around the shank, not the head, of the kingpin. Jaws closed on the head look connected and will pull apart under load.
Release arm properly seated and safety latch/lock engaged.
Upper Fifth Wheel
Glide plate securely mounted to the trailer frame.
Kingpin not damaged — no bends, cracks, or wear grooves.
Sliding Fifth Wheel
Slide not damaged and no parts missing.
Properly greased.
If air-powered, no air leaks.
All locking pins present and locked in place.
Not so far forward that the tractor frame will hit the landing gear, or the cab will hit the trailer, during turns. This is the check drivers skip after sliding for axle weights, and it produces expensive contact damage on the first tight turn.
2. Air and Electric Lines to the Trailer
Electrical cord firmly plugged in and secured.
Air lines properly connected to the glad hands, no air leaks, properly secured with enough slack for turns. Too little slack and the lines part in a sharp turn; too much and they drag or catch.
All lines free from damage — no cuts, abrasions, kinks, or chafe points.
3. Landing Gear
Fully raised , with no missing parts , not bent or otherwise damaged .
Crank handle in place and secured.
If power operated, no air or hydraulic leaks.
4. Shut-Off Valves (Cut-Out Cocks)
Shut-off valves sit in the service and supply air lines at the back of trailers used to tow other trailers. They allow the lines to be closed off when another trailer is not being towed.
Check that all shut-off valves are in the OPEN position, except the ones at the back of the LAST trailer, which must be CLOSED.
A closed valve in the middle of the string starves every trailer behind it of air — the brakes back there simply do not work. An open valve at the very back dumps the system to atmosphere.
5. The Combination Vehicle Brake Check
Do these checks in addition to the standard air brake system inspection. Perform them in order.
Step 1 — Check That Air Flows to All Trailers
Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red trailer air supply knob to supply air to the emergency (supply) lines. Use the trailer hand brake to provide air to the service line. Go to the rear of the rig and open the emergency line shut-off valve at the rear of the last trailer — you should hear air escaping , showing the entire system is charged. Close that valve. Then open the service line valve to check that service pressure reaches all the trailers (this assumes the trailer hand brake or the service brake pedal is applied), and close it.
If you do NOT hear air escaping from both lines , check that the shut-off valves on the trailers and dollies are in the OPEN position. You must have air all the way to the back for all the brakes to work.
Step 2 — Test the Tractor Protection Valve
Charge the trailer air brake system (build normal air pressure and push the air supply knob in). Shut the engine off. Step on and off the brake pedal several times to reduce tank pressure. The trailer air supply control — also called the tractor protection valve control — should pop out , or move from "normal" to "emergency," when pressure falls into the manufacturer's specified range, usually 20 to 45 psi .
If the tractor protection valve does not work right, an air hose or trailer brake leak could drain all the air from the tractor , causing the emergency brakes to come on with possible loss of control .
Step 3 — Test the Trailer Emergency Brakes
Charge the trailer air brake system and check that the trailer rolls freely . Then stop and pull out the trailer air supply control (tractor protection valve control / trailer emergency valve), or place it in the "emergency" position. Pull gently on the trailer with the tractor to confirm the trailer emergency brakes are applied.
Step 4 — Test the Trailer Service Brakes
Check for normal air pressure , release the parking brakes , move the vehicle forward slowly , and apply the 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 .
Important: 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. Using the hand valve to slow the vehicle on the road applies only the trailer brakes and is a classic cause of trailer swing .
6. Summary Table
Check What you do Pass criterion
Air to all trailers Open emergency then service line valve at rear of last trailer Air audibly escapes from both lines
Tractor protection valve Charge, engine off, fan the brakes down Trailer air supply control pops out, usually 20–45 psi
Trailer emergency brakes Pull out the trailer air supply control, tug gently with the tractor Trailer does not move