13.4 Inspecting Combination Vehicles: Walk-Around Additions and the Combination Vehicle Air Brake Check

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