Section 6: Combination Vehicles

This section is for drivers who need a Class A CDL

This section provides information needed to pass the test for Class A combination vehicles (tractor-trailers, or straight truck and trailer). The information gives you the minimum knowledge needed for driving most combination vehicles.

You should also study Section 7 if you need to pass the tests for doubles/triples.


Driving Combination Vehicles Safely

Combination vehicles are heavier, longer, and require more driving skill than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles. In this section, we list some important safety factors that apply specifically to combination vehicles.

Rollover Risks

More than half of truck driver deaths in collisions are from truck rollovers. As more cargo is stacked in a truck, the center of gravity gets higher from the road. The truck becomes easier to turn over. Fully loaded rigs are 10 times more likely to roll over in a collision than empty rigs.

The following two things will help to prevent rollovers: keep the cargo as close to the ground as possible, and go slowly around turns. Section 3 of this handbook talks about transporting cargo safely. Keeping cargo low is even more important in combination vehicles than in straight trucks. A trailer rollover is more likely if the load is to one side. Make sure your cargo is centered and spread out as much as possible.

Rollovers happen when you turn too fast. Go slowly around corners, onramps, and offramps. Avoid quick lane changes, especially when fully loaded. A tractor-trailer vehicle combination is most likely to roll over in a turn when the configuration includes triple 27 ft. trailers.

Steer Gently

Trucks with trailers have a dangerous "crack-thewhip" effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many collisions where only the trailer has overturned.

Steer carefully when you are pulling trailers. If you make a sudden movement with your steering wheel you could tip over a trailer. Follow far enough behind other vehicles (at least one second for each ten feet of vehicle length, plus another second if going over 40 mph). Look far enough down the road to avoid being surprised and having to make a sudden lane change. At night, drive slowly enough to see obstacles before it is too late to change lanes or stop gently. Slow down to a safe speed before going into a turn.

Brake Early

Control your speed whether fully loaded or empty. Large combination vehicles that are empty take longer to stop than when they are fully loaded. When lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels. When the wheels lock, your trailer can swing out and strike other vehicles or it can jackknife very quickly (Figure 6-1). You also must be very careful about driving "bobtail" tractors (tractors without semitrailers). Tests have shown that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor and semitrailer loaded to maximum gross weight.

In any combination rig, allow adequate following distance and look far enough ahead so you can brake early. Do not be caught by surprise and have to make a panic stop.

a diagram of a tractor jackknifed with the rear tractor wheels locked up or spinning and showing line of travel and direction on slide a diagram of a trailer jackknifed with trailer wheels locked and sliding and showing line of travel

Avoid Trailer Skids

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a "trailer jackknife." This is shown in Figure 6-2. The procedure for stopping a trailer skid is as follows:

  • Recognize the skid. The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors. Any time you apply the brakes hard, check the mirrors to make sure the trailer is staying where it should be. Once the trailer swings out of your lane, it is very difficult to prevent a jackknife.
  • Stop using the brake. Release the brakes to get traction back. Do not use the trailer hand brake to straighten out the rig. This is the wrong thing to do since it is the brakes on the trailer wheels that caused the skid in the first place. Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out. The best way to stop any skid is to get off the brakes and let the tires restore traction.
Make Wide Enough Turns

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is called offtracking. Figure 6-3 shows how offtracking causes the path followed by a tractor and semitrailer to be wider than the rig itself. Longer vehicles will offtrack more. The rear wheels of the powered unit (truck or tractor) will offtrack some, and the rear wheels of the trailer will offtrack even more. If there is more than one trailer, the rear wheels of the last trailer will offtrack the most. Steer the front end wide enough around a corner so the rear end does not run over the curb, pedestrians, other vehicles, etc. However, keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right. If you cannot complete your turn without entering another lane of traffic, turn wide as you complete the turn. This is better than swinging wide to the left before starting the turn because it will keep other drivers from passing you on the right.

a diagram of tractor/trailer off tracking in a 90 degree turn

Railroad-Highway Crossings

Railroad-highway crossings can also cause problems, particularly when pulling trailers with low underneath clearance.

  • These trailers can get stuck on raised crossings:
    • Low slung units (lowboy, car carrier, moving van, possum-belly livestock trailer).
    • Single-axle tractor pulling a long trailer with its landing gear set to accommodate a tandem-axle tractor.
  • If for any reason you get stuck on the tracks, get out of the vehicle and away from the tracks. Check sign posts or signal housing at the crossing for emergency notification information. Call 911 or other emergency number. Give the location of the crossing using all identifiable landmarks, especially the DOT number, if posted.
Backing Safely

Because you cannot see everything behind your vehicle, backing is always dangerous. Avoid backing whenever you can. When you park, try to park so you will be able to pull forward when you leave. When you have to back, here are a few simple safety rules:

  • Look at your path.
  • Back slowly, using your mirrors.
  • Back and turn toward the driver's side whenever possible.
  • Use a helper whenever possible.

Start in the proper position. Put the vehicle in the best position to allow you to back safely. This position will depend on the type of backing to be done.

Look at your path. Look at your line of travel before you begin. Get out and walk around the vehicle. Check your clearance to the sides and overhead in and near the path your vehicle will take.

Use mirrors on both sides. Check the outside mirrors on both sides frequently. Get out of the vehicle and check your path if you are unsure.

Back slowly. Always back as slowly as possible. Use the lowest reverse gear so that you can easily correct any steering errors before you get too far off course. You can also stop quickly if necessary.

Back and turn toward the driver's side. Back to the driver's side so you can see well. Backing toward the right side is very dangerous because you cannot see as well. Remember to always back in the direction that gives you the best vision.

Backing with a Trailer

Backing with a trailer. When backing a car, straight truck, or bus, turn the steering wheel toward the direction you want to go. When backing a trailer, turn the steering wheel in the opposite direction. Once the trailer starts to turn, you must turn the wheel the other way to follow the trailer.

  • Whenever you back with a trailer, try to position your vehicle so you can back in a straight line. If you must back on a curved path, back to the driver's side so you can see.
  • Back slowly so you can make corrections before you get too far off course.

Correct drift immediately. As soon as you see the trailer getting off the proper path, correct it by turning the steering wheel in the direction of the drift.

Pull forward. When backing, make pull-ups to reposition your vehicle when needed.

Use a helper. Use a helper when you can. He or she can see blind spots that you can't. The helper should stand near the back of the vehicle where you can see him or her. Before you begin backing, work out a set of hand signals that you both understand. Agree on a signal for STOP.


Combination Vehicle Air Brakes

You should study Section 5: Air Brakes before reading this. In combination vehicles the braking system has parts to control the trailer brakes, in addition to the parts described in Section 5. These parts are described below.

Trailer Hand Valve
  • The trailer hand valve (also called the trolley valve or Johnson bar) works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all of the brakes on the vehicle (including the trailer(s)). There is much less danger of causing a skid or jackknife when using just the foot brake.
  • Never use the hand valve for parking because all the air might leak out unlocking the brakes (in trailers that don't have spring brakes). Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.
Tractor Protection Valve

The tractor protection valve keeps air in the tractor or truck if the trailer breaks away or develops a bad leak. The tractor protection valve is controlled by the trailer air supply control valve in the cab. The control valve allows you to open and shut the tractor protection valve. It will close automatically if air pressure is low (in the range of 20 to 45 psi). When the valve closes, it stops any air from escaping and lets the air out of the trailer emergency line which causes the trailer emergency brakes to come on. (Emergency brakes are covered later.)

Trailer Air Supply Control

The trailer air supply control on newer vehicles is a red eight-sided knob, which you use to control the tractor protection valve. You push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out (thus closing the tractor protection valve) when the air pressure drops between 20 to 45 psi. Tractor protection valve controls or "emergency" valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The "normal" position is used for pulling a trailer. The "emergency" position is used to shut the air off and put on the trailer emergency brakes.

Trailer Air Lines

Every combination vehicle has two air lines—the service line and the emergency line. They run between each vehicle (tractor to trailer, trailer to dolly, dolly to second trailer, etc.).

  • Service air line (normally blue). The service line (also called the control line or signal line) carries air which is controlled by the foot brake or the trailer hand brake. The pressure in the service line will similarly change depending on how hard you press the foot brake or hand valve. The service line is connected to a relay valve on the trailer to apply more or less pressure to the trailer brakes. The relay valve connects the trailer air tanks to the trailer air brakes. As pressure builds up in the service line, the relay valve opens and sends air pressure from the trailer air tank to the trailer brake chambers, putting on the trailer brakes.
  • Emergency air line (normally red). The emergency line has two purposes. First, it supplies air to the trailer air tanks and secondly, the emergency line controls the emergency brakes on combination vehicles. Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The pressure loss could be caused by a trailer breaking loose, tearing apart the emergency air hose. It could also be caused by a hose, metal tubing, or other part which breaks, letting the air out. When the emergency line loses pressure, it causes the tractor protection valve to close (the air supply knob will pop out).
Trailer Air Tanks

Each trailer and converter dolly has one or more air tanks. They are filled by the emergency (supply) line from the tractor. They provide the air pressure used to operate trailer brakes. Air pressure is sent from the air tanks to the brakes by relay valves.

The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The pressure in the service line is controlled by the brake pedal (and the trailer hand brake).

It is important that you don't let water and oil build up in the air tanks. If you do, the brakes may not work correctly. Each tank has a drain valve on it and you should drain each tank every day. If your tanks have automatic drains, they will keep most moisture out. But you should still open the drains to make sure.

Shut-Off Valves

Shut-off valves (also called cut-out cocks) are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must 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.

Trailer Service, Parking and Emergency Brakes

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes, which work from the air stored in the trailer air tank. The emergency brakes come on whenever air pressure in the emergency line is lost. These trailers have no parking brake. The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected. A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on. The brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away and then there will be no brakes. Therefore, it is very important for safety that you use wheel chocks when you park trailers without spring brakes.

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

Hose Couplers (Glad Hands)

Glad hands are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. The couplers have a rubber seal which prevents air from escaping. Clean the couplers and rubber seals before a connection is made. When connecting the glad hands, press the two seals together with the couplers at a 90° angle to each other. A turn of the glad hand attached to the hose will join and lock the couplers.

It is very important to keep the air supply clean. To keep the air supply clean, some vehicles have "dead end" or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air lines. Use the dummy couplers, if available, when the air lines are not connected to a trailer.

To avoid mistakes, metal tags are sometimes attached to the lines with the words service or emergency stamped on them. Sometimes colors are used. Blue is used for the service lines and red for the emergency lines.

If you do cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. Air will not be available to release the trailer spring brakes (parking brakes). If the spring brakes don't release when you push the trailer air supply control, check the air line connections.

Older trailers do not have spring brakes. If the air supply in the trailer air tank has leaked away there will be no emergency brakes and the trailer wheels will turn freely. If you crossed the air lines, you could drive away but you would not have trailer brakes. Before driving, always test the trailer brakes with the hand valve or by pulling the air supply control. Pull gently against them in a low gear to make sure the brakes work.

In-Cab Air Brake Check

Note: All the Air Brakes system tests in this section are considered important and each can be considered critical parts of the in-cab air brakes tests. The items marked with an asterisk (*) in this section are required for testing purposes during the pre-trip portion of the CDL driving test. They may be performed in any order as long as they are performed correctly and effectively. If these items are not demonstrated and the parameters for each test are not verbalized correctly, it is considered an automatic failure of the pre-trip portion of the test.

Testing air leakage rate. There are two tests as follow:

Static Leakage Test

With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for one minute. The air pressure should not drop more than:

  • 2 psi for single vehicles.
  • 3 psi for a combination of two vehicles.
  • 5 psi for a combination of three or more vehicles.

An air loss greater than those shown indicate a problem in the braking system and repairs are needed before operating the vehicle.

*Applied Leakage Test

With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes so the entire system is charged. Allow the system to settle (air gauge needle stops moving), apply firm, steady pressure to the brake pedal (brake on), and hold. After the system settles again, time for one minute. The air pressure should not drop more than:

  • 3 psi for single vehicles.
  • 4 psi for a combination of two vehicles.
  • 6 psi for a combination of three or more vehicles.

An air loss greater than those shown indicate a problem in the braking system and repairs are needed before operating the vehicle.

Note: You must be able to demonstrate this test and verbalize the allowable air loss for the examiner on this test.

If the air loss is too much, check for air leaks and fix. For testing purposes, identify if the air loss rate is too much.

*Air Compressor Governor Cut-Out Pressure Test

To perform this test, the air pressure for the vehicle must be rising when the engine is running. Run the engine at a fast idle. The air compressor governor must cut-out prior to the needle reaching 130 psi. Where the needle stops rising is the governor cut-out pressure.

For testing purposes, identify where the air governor cuts out the compressor and verbalize the maximum pressure at which this can occur.

Note: The air dryer exhausting should not be referenced as governor cut-out.

*Air Compressor Governor Cut-In Pressure Test

To perform this test, the air pressure for the vehicle cannot be rising when the engine is running. With the engine idling, slowly pump the brake pedal to reduce the air tank pressure. Watch the air pressure gauge between pumps to identify when the compressor cuts in (needle starts to rise). This should occur no lower than 85 psi.

For testing purposes, identify where the air governor cuts in the compressor and verbalize the minimum pressure at which this can occur.

*Low Air Pressure Warning Device Test

This test may be performed with engine on or off. To perform the test with the engine off, turn the electrical power on and have enough air pressure to keep the low air pressure warning device from coming on. Slowly pump the brake pedal to reduce air tank pressure. The low air pressure warning device must activate between 55 and 75 psi. For testing purposes, identify when the warning signal activates, and verbalize the legal range in which the signal must activate.

If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

*Check that the spring brakes come on automatically. Chock the wheels. Release all parking brakes and shut the engine off. Pump the brake pedal to reduce the air tank pressure. The trailer air supply valve knob and tractor protection valve should pop out when the air pressure falls to the manufacturer's specifications (usually in a range between 20 to 45 psi). This causes the spring brakes to engage. Some trailers use an air applied emergency brake system and some trailers use spring brakes as the emergency brake system.

Check rate of air pressure buildup. With the engine at operating rpms, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer's specifications. In single air systems (pre-1975), typical requirements are pressure buildup from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 rpms.

If air pressure does not build up fast enough, the pressure may drop too low during driving, requiring an emergency stop.

Test service brakes. Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph), and apply the brakes firmly using the brake pedal. Any pulling to one side, unusual feel, or delayed stopping action should be checked.

Test parking brake. Fasten your seat belt. Set the parking brake and try to move the vehicle or allow the vehicle to slowly move forward and apply the parking brake. The parking brake should stop a rolling vehicle, or not allow any movement.


Antilock Brake Systems

Trailers Required to Have ABS
  • All trailers and converter dollies built on or after March 1, 1998, are required to have ABS. However, many trailers and converter dollies built before this date have been voluntarily equipped with ABS.
  • Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner. Converter dollies manufactured on or after March 1, 1998, are required to have a lamp on the left side.
  • In the case of vehicles manufactured before the required date, it may be difficult to tell if the unit is equipped with ABS. Look under the vehicle for the ECU and wheel speed sensor wires coming from the back of the brakes.
Braking with ABS
  • ABS is an addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only activates when wheels are about to lock up.
  • ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.
  • ABS helps you avoid wheel lock up. The computer senses impending lockup, reduces the braking pressure to a safe level, and you maintain control.
  • Having ABS on only the trailer, or even on only one axle, still gives you more control over the vehicle during braking.
  • When only the trailer has ABS, the trailer is less likely to swing out, but if you lose steering control or start a tractor jackknife, let up on the brakes (if you can safely do so) until you gain control.
  • When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:
    • Use only the braking force necessary to stop safely and stay in control.
    • Brake the same way, regardless of whether you have ABS on the tractor, the trailer, or both.
    • As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.
  • Remember, if your ABS malfunctions, you still have regular brakes. Drive normally, but get the system serviced soon.
  • ABS won't allow you to drive faster, follow more closely, or drive less carefully.

Coupling and Uncoupling

Knowing how to couple and uncouple correctly is basic to safe operation of combination vehicles. Coupling and uncoupling incorrectly can be very dangerous. There are differences between different rigs, so learn the details of coupling and uncoupling the vehicle(s) you will operate. General coupling and uncoupling steps are listed below:

Coupling Tractor/Semitrailers

Step 1. Inspect the Fifth-Wheel

  • For damaged or missing parts.
  • To see that mounting to tractor is secure, no cracks in frame, etc.
  • To see that the fifth-wheel plate is completely greased. Failure to keep the fifth-wheel plate lubricated could cause steering problems because of friction between the tractor and trailer.
  • To see that the fifth-wheel is in proper position for coupling.
    • the fifth-wheel should be tilted down towards the rear of the tractor with the jaws open and the safety unlocking handle in the automatic lock position.
  • To see that the sliding fifth-wheel is locked.
  • To see that the trailer kingpin is not bent or broken.

Step 2. Inspect Area and Chock Wheels

  • To be sure the area around the vehicle is clear.
  • To be sure the trailer wheels are chocked or the spring brakes are on.
  • To see that cargo (if any) is secured against movement during coupling.

Step 3. Position Tractor

  • Directly in front of the trailer. (Never back under the trailer at an angle because you might push the trailer sideways and break the landing gear.)
  • Check position, using outside mirrors, by looking down both sides of the trailer.

Step 4. Back Slowly

  • Until the fifth-wheel just touches the trailer.
  • Do not hit the trailer.

Step 5. Secure the Tractor

  • Set the parking brake.
  • Put the transmission in neutral.

Step 6. Check the Trailer Height

  • The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed.
  • To see that the kingpin and fifth-wheel are aligned.

Step 7. Connect the Air Lines to the Trailer

  • Check glad hand seals and connect tractor emergency air line to trailer emergency glad hand.
  • Check glad hand seals and connect tractor service air line to trailer service glad hand.
  • Make sure air lines are safely supported where they won't be crushed or caught while tractor is backing under the trailer.

Step 8. Supply Air to the Trailer

  • From the cab, push in the air supply knob or move tractor protection valve control from the "emergency" to the "normal" position to supply air to the trailer brake system.
  • Wait until the air pressure is normal. Check brake system for crossed air lines:
    • shut engine off so you can hear the brakes.
    • apply and release trailer brakes and listen for sound of trailer brakes being applied and released. You should hear the brakes move when applied and air escape when the brakes are released.
    • check air brake system pressure gauge for signs of major air loss.
  • When trailer brakes are working, start the engine.
  • Air pressure must be up to normal.

Step 9. Lock the Trailer Brakes

  • Pull out the air supply knob or move the tractor protection valve control from normal to emergency.

Step 10. Back Under the Trailer

  • Use lowest reverse gear.
  • Back tractor slowly under trailer to avoid hitting the kingpin.
  • Stop when the kingpin is locked into the fifth-wheel.

Step 11. Check the Connection for Security

  • Raise the landing gear slightly off the ground.
  • Pull forward gently against the trailer brakes to be sure that the trailer is locked to the tractor.

Step 12. Secure the Vehicle

  • Put transmission in neutral.
  • Put parking brakes on.
  • Shut off the engine and take the key so someone will not move the truck.

Step 13. Inspect the Coupling

  • Use a flashlight, if necessary.
  • Make sure there is no space between the upper and lower fifth-wheel.
  • Make sure the fifth-wheel jaws have closed around the shank of the kingpin. (Figure 6-4)
  • Check that the locking lever is in the "lock" position.
  • Check that the safety catch is in position over the locking lever.

Step 14. Connect the Electrical Cord and Check Air Lines

  • Plug the electrical cord into the trailer and fasten the safety catch.
  • Check both air and electrical lines for signs of damage.
  • Make sure air and electrical lines will not hit any moving parts.

Step 15. Fully Raise the Front Trailer Supports (Landing Gear)

  • Use low gear range, if equipped, to begin raising the landing gear. Once free of weight, switch to the high gear range.
  • Raise the landing gear all the way up.
  • After raising landing gear, secure the crank handle safely.
  • When full weight of trailer is resting on tractor:
    • check for clearance between the rear of the tractor frame and the landing gear.
    • check for clearance between the top of the tractor tires and the nose of the trailer.

Step 16. Remove and Store the Trailer Wheel Chocks

a diagram of trailer kingpin viewed from underneath the trailer
Uncoupling Tractor/Semitrailer

The following steps will help you to uncouple safely:

Step 1. Position the Rig

  • Make sure surface of parking area can support weight of trailer.
  • Have tractor lined up with the trailer.

Step 2. Ease the Pressure on the Locking Jaws

  • Shut off trailer air supply to lock trailer brakes.
  • Ease pressure on fifth-wheel locking jaws by backing up gently.
  • Put parking brakes on while tractor is pushing against the kingpin.

Step 3. Chock the Trailer Wheels

  • Chock the trailer wheels if the trailer doesn't have spring brakes or if you are not sure.

Step 4. Lower the Landing Gear

  • If trailer is empty—lower the landing gear until it makes firm contact with the ground.
  • If trailer is loaded, after the landing gear makes firm contact with the ground, turn crank in low gear a few extra turns. This will lift some weight off the tractor. This will:
    • make it easier to unlatch fifth-wheel.
    • make it easier to couple next time.

Step 5. Disconnect the Air Lines and Electrical Cable

  • Disconnect air lines from trailer. Connect air line glad hands to dummy couplers at back of cab or couple them together.
  • Hang electrical cable with plug down to prevent moisture from getting in.
  • Make sure lines are supported so they won't be damaged while driving the tractor.

Step 6. Unlock the Fifth-Wheel

  • Raise the release handle lock.
  • Pull the release handle to "open" position.
  • Keep legs and feet clear of the rear tractor wheels to avoid serious injury.

Step 7. Pull the Tractor Partially Clear of the Trailer

  • Pull tractor forward until fifth-wheel comes out from under the trailer.
  • Stop with tractor frame under trailer.

Step 8. Secure the Tractor

  • Apply parking brake.
  • Place transmission in neutral.

Step 9. Inspect the Trailer Supports

  • Make sure ground is supporting trailer.
  • Make sure landing gear is not damaged.

Step 10. Pull the Tractor Clear of the Trailer

  • Release parking brakes.
  • Check the area and drive tractor forward until it clears.

Inspecting a Combination Vehicle

Use the inspection procedures described in Sections 2 and 11 to inspect your combination vehicle. However, there are more items to inspect on a combination vehicle than on a single vehicle.

In addition to the checks already listed in Section 2, complete these checks:

Additional Items for Walkaround Inspection

Coupling system areas:

  • Fifth-wheel (lower):
    • securely mounted to frame
    • no missing, damaged parts
    • properly greased
    • no visible space between upper and lower fifth-wheel
    • locking jaws around the shank, not the head of the kingpin
    • release arm properly seated and safety latch/lock engaged
  • Fifth-wheel (upper):
    • glide plate securely mounted to trailer frame
    • kingpin not damaged
  • Air and electric lines to trailer:
    • electrical cord firmly plugged in and secured
    • air lines properly connected to glad hands, no air leaks, properly secured with enough slack for turns
    • all lines free from damage
  • Sliding fifth-wheel:
    • slide not damaged or parts missing
    • properly greased
    • all locking pins present and locked in place
    • if air powered—no air leaks
    • fifth-wheel not so far forward that tractor frame will hit landing gear, or cab hit the trailer, during turns

Landing gear:

  • Fully raised, no missing parts, not bent or otherwise damaged.
  • Crank handle in place and secured.
  • If power operated, no air or hydraulic leaks.
Combination Vehicle Brake Check

Do these checks in addition to Section 5: Inspecting Air Brake System.

The following section explains how to check air brakes on combination vehicles. Check the brakes on a double or triple trailer as you would any combination vehicle.

  • 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. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. 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 the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), and then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly(ies) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.
  • Test tractor protection valve. Charge the trailer air brake system. (That is, build up 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 the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out (or go from "normal" to "emergency" position) when the air pressure falls into the pressure range specified by the manufacturer. (Usually within the range of 20 to 45 psi.)
    • If the tractor protection valve doesn't work right, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control.
  • Test 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 (also called tractor protection valve control or trailer emergency valve), or place it in the "emergency" position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.
  • Test trailer service brakes. Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply 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. (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.)


Section 5: Air Brakes | Table of Contents | Section 7: Doubles and Triples