Commercial Driver's License (CDL) Air Brakes and Combination Vehicles Syllabus

At what air pressure do the spring (emergency) brakes typically come on automatically?

On tractors and straight trucks, the spring brakes will come on fully when air pressure drops to a range of about 20 to 45 psi (typically 20-45 psi). A low-air warning should activate well before this.

How are the parking brakes applied and released in a vehicle with spring brakes?

You apply the parking brakes by pulling out the yellow, diamond-shaped parking brake control knob, which releases air and lets the springs apply the brakes. You release them by pushing the knob in, sending air pressure to hold the springs back.

Why should you never push the brake pedal down (apply the service brakes) when the spring (parking) brakes are already on?

If the spring brakes are on and you also press the brake pedal, the combined force of the springs and the air pressure could damage the brakes by applying too much force. Make sure the system has enough air before releasing the parking brakes.

At what air pressure must the low-air-pressure warning signal come on, per regulation?

The low-air-pressure warning signal must come on before the air pressure in the tanks falls below 60 psi (it may activate at a higher pressure on some vehicles). It is usually a red light and often a buzzer.

What is the purpose of the supply pressure (air pressure) gauge?

The air pressure gauge shows how much pressure is in the air tanks. It lets the driver monitor that the system has enough air to operate the brakes safely.

What is the application pressure gauge, and what does a rapid rise during braking indicate?

The application pressure gauge shows how much air pressure you are applying to the brakes (not all vehicles have one). If it takes increasing pressure to maintain the same speed, it warns that the brakes are fading and you should slow down and use a lower gear.

What is a dual air brake system, and why is it used?

A dual air brake system has two separate air brake systems (a primary and a secondary) that use a single set of brake controls. It provides redundancy so that if one system fails, the other can still stop the vehicle.

In a typical dual air brake system, which axles does each system serve?

Typically, one system (the primary) controls the rear axle(s) and the other (the secondary) controls the front axle; the exact split varies by vehicle. Each system has its own tanks, lines, and hoses.

Before driving a vehicle with a dual air brake system, to what pressure should you build the air, and what should you watch?

Allow time for the air compressor to build up a minimum of 100 psi in both the primary and secondary systems. Watch the warning light/buzzer to ensure it shuts off (it should go off by 60 psi or above) before driving.

In a dual air brake system, what happens if one of the two systems is very low or out of air?

The low-air-pressure warning light/buzzer activates. The remaining system can still stop the truck, but braking is reduced. You should bring the vehicle to a safe stop and have the air brakes repaired.

How do you test the low-air-pressure warning signal during a pre-trip inspection?

With the engine off and enough air pressure, turn the key to the on/accessory position. Step on and off the brake pedal to reduce air tank pressure. The low-air warning light and buzzer must come on before pressure drops below 60 psi.

How do you check that the spring brakes come on automatically?

Continue to step on and off the brake pedal to reduce tank pressure further. The parking brake knob should pop out (or the tractor protection valve close) by the time pressure falls to about 20-45 psi, applying the spring brakes automatically.

How do you test the air compressor's governor cut-in and cut-out pressures?

Run the engine at fast idle. The air governor should stop the compressor from pumping (cut-out) at about 125 psi. Then with the engine at idle, step on and off the brake to reduce pressure; the compressor should start pumping again (cut-in) at about 100 psi.

How do you test the air leakage (static air loss) rate, and what is the maximum allowable rate for a single vehicle?

With a fully charged system (about 125 psi), turn off the engine, release (chock) and then with brakes released time the pressure drop. For a single vehicle (straight truck/bus), air loss should be less than 2 psi in one minute.

What is the maximum allowable static air loss rate for a combination vehicle with brakes released?

For a combination vehicle (e.g., tractor-trailer), the air loss rate should be less than 3 psi in one minute with the brakes released.

What is the maximum allowable air loss rate with the brakes APPLIED, for a single vehicle and a combination vehicle?

With the brakes applied (a hard application held), air loss should be less than 3 psi in one minute for a single vehicle and less than 4 psi in one minute for a combination vehicle.

What is brake lag, and why is it important in air brake systems?

Brake lag is the time it takes for the air to flow through the lines and apply the brakes after you press the pedal—about 0.5 second. Because of this delay, air-braked vehicles need extra total stopping distance compared with hydraulic brakes.

What are the three components of total stopping distance for an air-braked vehicle?

Total stopping distance = perception distance + reaction distance + braking distance. Air brakes add a fourth factor: brake lag distance (the distance traveled during the roughly half-second brake lag).

How do you check air buildup (compressor build-up time) in a dual system during inspection?

With the engine at operating rpm, the air pressure should build from about 85 to 100 psi within about 45 seconds in dual air systems. If it takes longer, pressure may drop too low during driving, requiring an emergency stop.

What is rearward amplification (the crack-the-whip effect), and which vehicles are most affected?

Rearward amplification is the increasing side-to-side movement that is amplified from the tractor back through each trailer, like cracking a whip. Multi-trailer combinations (doubles and triples) experience the most rearward amplification; the last trailer is most likely to roll over.

Roughly how much greater is the rearward amplification on the rear trailer of a double, and what does this mean for driving?

A quick steering movement can be amplified so the rear trailer's reaction is several times (often cited as up to about 2 times for a 5-axle double, more for triples) the tractor's movement. Steer gently and gradually, and allow more room to maneuver to avoid rolling the rear trailer.

What is the leading cause of truck rollover, and at what point can a trailer roll over?

Most rollovers happen because the driver takes a turn or curve too fast. A loaded trailer can roll over at less than half the g-force needed to roll over a passenger car, and a trailer with a high center of gravity can roll over even when the tractor stays upright.

How do load placement and tire inflation affect rollover risk in combination vehicles?

A high center of gravity (heavy cargo loaded high or unevenly) makes the vehicle more likely to tip. Keeping the load low and centered, securing it, and going slow around turns lowers the center of gravity effect and reduces rollover risk.

What is trailer jackknife, and what causes the trailer to swing out?

A trailer jackknife occurs when the trailer's wheels lock up and the trailer skids, causing the trailer to swing out to one side and fold toward the tractor like a folding pocketknife. Rear (trailer) wheel braking skids are the major cause.

How do you correct a trailer skid (trailer swing) caused by locked trailer wheels?

Recognize it by checking your mirrors—any time you apply the brakes hard and the trailer swings out, you are skidding. Stop braking: release the brakes to let the trailer wheels start rolling again and regain traction so the trailer can straighten out.

What is a drive-wheel (tractor) skid, and how do you correct a front-wheel braking/steering skid?

A drive-wheel skid (often on ice/snow) causes the tractor to spin; correct it by stopping braking to let the wheels roll, turning quickly toward the skid direction, and countersteering. A front-wheel skid causes loss of steering control; correct it by letting up on the brakes so the front wheels grip again.

Why is a combination vehicle harder to control when EMPTY than when loaded?

Empty trucks have less traction because there is little weight on the drive and trailer axles; the wheels lock up more easily during hard braking, increasing the chance of skids and jackknifing. Empty rigs can also bounce, and empty stopping distance can actually be longer than partly loaded.

Why must you start braking earlier and brake gently with an empty combination unit on slippery roads?

With little weight over the axles, hard braking quickly locks the wheels of an empty rig, causing skids and loss of control. Brake earlier and more gently, and leave more following distance, to avoid wheel lockup.

What is off-tracking (cheating) in a combination vehicle, and how do you compensate when turning?

Off-tracking means the rear wheels of a trailer follow a tighter (shorter) path than the front wheels in a turn, so the trailer cuts the corner. To compensate, swing wide by turning the tractor wider as you complete the turn (turn wide at the end, not the beginning) so the trailer clears curbs and other vehicles.

List the steps to couple a tractor and semitrailer (fifth wheel/kingpin coupling).

(1) Inspect the fifth wheel and kingpin; (2) chock trailer wheels or set trailer brakes and back tractor squarely in front of the trailer (never back under at an angle); (3) back slowly until the fifth wheel just touches the trailer; (4) secure the tractor (brakes/gear); (5) connect the air lines (glad hands) and electrical cord; (6) supply air to the trailer system; (7) back under the trailer until the kingpin locks into the fifth wheel; (8) raise the landing gear, then perform a coupling check.

What is the proper fifth-wheel height for coupling, and why does correct height matter?

The trailer should be at a height where it will be raised slightly (about 1/2 inch) when the tractor backs under it; the fifth wheel should be tilted down toward the rear of the tractor. Correct height prevents the kingpin from sliding over the top of the fifth wheel or missing the locking jaws.

How do you connect the glad hands (air lines) and avoid cross-connecting service and emergency lines?

Clean the glad hand rubber seals, then press the two together with the couplers at a 90-degree angle and turn one to lock. The service (control) line and emergency (supply) line are often color-coded (blue=service, red=emergency) and may have different-shaped connectors; connect the correct line to each so the trailer brakes work properly.

How do you verify the kingpin is securely locked in the fifth wheel after backing under?

Do a tug test: with the trailer brakes locked, gently pull the tractor forward in low gear against the locked fifth wheel; the trailer should not move. Then go under the trailer and visually inspect: the locking jaws must be closed around the shank of the kingpin (not the head), and the fifth wheel must be flush against the trailer with no gap and no space between the upper and lower fifth wheel.

What is the proper way to inspect the coupling for security under the trailer?

Use a flashlight if needed. Check that there is no space between the upper and lower fifth wheel, the locking jaws are fully closed around the kingpin shank, and the release/safety latch (locking lever) is in the locked position with the safety catch over it. Make sure the kingpin is not bent or broken.

When raising the landing gear after coupling, what must you check?

Raise the landing gear fully (never leave it part way up—it could catch on railroad tracks or pavement) and secure the crank handle. After full raising, make sure there is enough clearance between the rear of the tractor frame and the landing gear, and between the tractor and the trailer's front so they won't hit during turns.

List the proper steps to uncouple a trailer safely.

(1) Position the rig in a straight line; (2) ease the pressure off the fifth wheel locking jaws (back up gently against locked trailer brakes); (3) chock the trailer wheels if the trailer has no spring brakes; (4) lower the landing gear until it just touches the ground, then a few more turns to lift some weight off the tractor; (5) disconnect the air lines and electrical cable and secure them; (6) unlock the fifth wheel (pull the release handle to the open position); (7) pull the tractor partway out from under the trailer until the fifth wheel clears the apron, then secure the tractor and finish pulling clear.

Before lowering the landing gear when uncoupling, why must you make sure the trailer is supported and the surface is firm?

The landing gear (trailer support) must hold the trailer's weight once the tractor pulls out. If the ground is soft or the landing gear is lowered onto an uneven surface, the trailer can fall; make sure the surface supports the weight and lower the landing gear until it bears some weight before disconnecting.

Why should you chock the trailer wheels before uncoupling if the trailer has no spring brakes?

If the trailer has no spring (emergency) brakes, the trailer brakes are held only by air pressure that is lost when you disconnect the air lines. Without chocks the trailer could roll, so chock the wheels to keep it in place during and after uncoupling.

What is the tractor protection valve, and what does it do if the trailer breaks away or a severe air leak occurs?

The tractor protection valve keeps air in the tractor's brake system if the trailer breaks away or develops a bad air leak. It closes automatically (or can be closed by the trailer air supply valve) when pressure drops to about 20-45 psi, protecting the tractor's air supply and triggering the trailer's emergency brakes.

What is the trailer air supply (red, octagonal) control valve used for?

The red, eight-sided trailer air supply knob controls the tractor protection valve. Pushing it in supplies air to the trailer (used for normal driving); pulling it out shuts off the air to the trailer and applies the trailer emergency brakes. It should pop out automatically when air pressure drops to the 20-45 psi range.

How do you test the trailer (service) brakes after coupling, and what does the check confirm?

With air pressure built up, release the parking brakes, move the vehicle forward slowly, and apply the trailer brakes by hand using the hand valve (trolley/Johnson bar) if equipped, or apply the foot brake. The vehicle should slow/stop, confirming the trailer service brakes are connected and working through the service air line.