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Inspecting Combination Vehicles

Combination vehicles have coupling and trailer components that must be inspected in addition to the normal tractor checks. This chapter covers the fifth wheel and coupling system, the trailer air lines and brakes, and the trailer's tires, lights, and suspension. A careful pre-trip inspection catches problems that could cause a separation, brake failure, or breakdown on the road.

Inspecting the Coupling System

The coupling system holds the tractor and trailer together, so it deserves close attention. Check the fifth wheel mounting, the kingpin and jaws, and the apron contact. Any looseness, gap, or damage here can lead to a trailer separation.

Check fifth wheel mounting

Confirm the fifth wheel is securely mounted with no missing or broken bolts, brackets, or welds.

Check the locking jaws

Verify the jaws are closed around the kingpin shank with no gap between the upper and lower plates.

Check the trailer apron

Make sure the trailer apron sits flat and fully on the fifth wheel skid plate with no gap or bending.

Check a sliding fifth wheel

If equipped, confirm the slide is locked in place and the locking pins are fully engaged, with no cracked rails.

Inspecting Air Lines and Electrical Connections

The service and emergency air lines carry the air that operates the trailer brakes, and the electrical cord powers the trailer lights. Damaged or leaking connections can cause loss of braking or lighting. Inspect them for wear, leaks, and secure attachment.

Inspect the glad hands

Check the service and emergency air lines for cuts, cracks, wear, or leaks and confirm they are securely connected.

Check for air leaks

Listen and feel at the connections; leaks reduce braking air and can lead to brake failure.

Secure the electrical cord

Confirm the light cord is firmly plugged in, latched if equipped, and not frayed or dragging.

Prevent chafing

Make sure lines are routed and supported so they do not rub on the frame or get pinched.

Inspecting the Trailer Braking System

Combination vehicles require checks of the trailer service brakes, emergency brakes, and the protection valves. These tests confirm the trailer will stop and that the tractor is protected if a trailer line fails. Perform them as part of the air brake check.

Test the service brakes

Charge the system, release parking brakes, roll slowly, and apply the pedal to confirm a straight stop with no pulling.

Test the emergency brakes

Charge the air, then pull the trailer air supply valve and confirm the trailer spring brakes set.

Test the protection valve

Confirm the tractor protection and trailer air supply valves close when pressure drops to a low level, roughly 20 to 45 psi.

Check slack adjusters

Confirm the trailer brakes are within proper adjustment so stopping power is not reduced.

Drain the air tanks

Drain moisture and oil from the trailer air tank so it does not damage the braking system.

Inspecting Trailer Tires, Wheels, and Suspension

Trailer tires and wheels are easy to overlook but are essential to safe operation. Inspect all trailer tires and wheels, and check the suspension that supports the load. Problems here can cause a blowout, wheel loss, or handling issues.

Check tire condition

Confirm proper inflation and tread and look for cuts, bulges, or uneven wear on all trailer tires.

Check wheels and lug nuts

Look for cracked rims and missing, loose, or damaged lug nuts, including rust streaks that indicate looseness.

Check the suspension

Inspect for cracked or broken spring hangers, missing spring leaves, and loose or leaking air suspension parts.

Check mounting hardware

Confirm U-bolts, shock absorbers, and axle mounting components are secure and undamaged.

Inspecting Trailer Body, Lights, and Load

Finally, walk around the trailer to check its lights, reflectors, doors, and cargo securement, along with landing gear and rear protection. These items keep the rig visible, legal, and safe. A missing light or shifting load can create a hazard for you and others.

Check lights and reflectors

Confirm clearance, marker, brake, tail, and turn lights work and that reflectors are present and clean.

Check landing gear

Confirm the landing gear is fully raised, the crank handle secured, and no parts are missing, bent, or damaged.

Check cargo securement

Confirm doors are latched and cargo is secured so nothing shifts or falls while driving.

Check mud flaps and rear guard

Verify the mud flaps and rear impact guard are present, secure, and undamaged.

Check load balance

Confirm the trailer is not overloaded or unbalanced, since improper weight affects braking and rollover risk.

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Last updated: July 2026