CDL Air Brakes — All Questions

What is the purpose of the air compressor in an air brake system?

• a.To release the parking brakes automatically
• b.To apply the service brakes evenly
• c.To warn the driver of low air pressure
• d.To pump air into the air storage tanks✓

The air compressor pumps air into the storage tanks (reservoirs) that hold the compressed air used for braking. It is usually connected to the engine by gears or a belt. Without it, the system could not build or maintain the air pressure the brakes need to work.

What does the air compressor governor control?

• a.The temperature of the compressed air
• b.The pressure applied to the brake drums
• c.The speed of the vehicle at highway speeds
• d.When the compressor pumps air into the tanks✓

The governor controls when the compressor pumps air into the storage tanks. When tank pressure reaches the cutout level (around 125 psi), the governor stops the compressor from pumping. When pressure falls to the cut-in level (around 100 psi), it allows the compressor to start pumping again.

At about what pressure does the governor typically cut out the compressor from pumping air?

• a.Around 125 psi✓
• b.Around 20 psi
• c.Around 200 psi
• d.Around 60 psi

The governor usually cuts out the compressor at about 125 psi, stopping it from pumping more air into the tanks. It cuts the compressor back in at about 100 psi to keep the system in the normal operating range. These values keep tank pressure within a safe working band.

Why do air storage tanks have a drain valve at the bottom?

• a.To remove water and compressor oil that collect inside✓
• b.To increase the air pressure faster
• c.To connect the trailer air lines
• d.To add oil to the compressed air

Compressed air contains some water and compressor oil that collect at the bottom of the tanks. This buildup can freeze in cold weather and damage the brakes, so tanks have drain valves to remove it. Tanks without automatic drains must be drained manually, often at the end of each driving day.

What is the danger of water collecting in the air storage tanks?

• a.It makes the compressor run too slowly
• b.It causes the low-pressure warning to stay on
• c.In cold weather it can freeze and cause brake failure✓
• d.It increases stopping distance on dry roads

Water collecting in the tanks can freeze in cold weather, and frozen moisture can keep the brakes from working properly. That is why tanks must be drained regularly to remove accumulated water and oil. Some vehicles have automatic drain valves, but manual drains should still be checked.

What is the function of the S-cam in a drum air brake?

• a.To store compressed air for emergencies
• b.To force the brake shoes against the drum✓
• c.To drain water from the air tanks
• d.To warn the driver when pressure is low

When you apply the brakes, air pressure moves the slack adjuster and brake chamber pushrod, which turns the S-cam. The S-cam forces the brake shoes apart and presses them against the inside of the brake drum to create stopping friction. This design is called an S-cam brake because of the cam's shape.

Spring brakes on a truck are held in the released position by what?

• a.Hydraulic fluid
• b.Electrical current
• c.Air pressure✓
• d.Engine vacuum

Spring brakes use powerful springs to apply the brakes, and air pressure holds those springs back so the brakes stay released while driving. If air pressure is lost, the springs push the brakes on automatically. This provides an automatic safety braking feature when air is lost.

What happens to the spring brakes when air pressure drops to a very low level (for example, 20 to 45 psi)?

• a.The springs apply the brakes automatically✓
• b.The compressor speeds up to rebuild pressure
• c.The service brakes are disabled entirely
• d.They release and the vehicle can roll freely

When air pressure falls into the range of about 20 to 45 psi, the spring brakes come on automatically because there is no longer enough air to hold the springs back. This can bring the vehicle to a sudden stop, so a driver must safely control the vehicle if this begins to happen. It is a fail-safe designed to stop a vehicle that loses air.

Why do heavy vehicles use a dual air brake system?

• a.To allow the compressor to run continuously
• b.So that if one system leaks, the other can still stop the vehicle✓
• c.To make the brakes work with less pressure
• d.To eliminate the need for spring brakes

A dual air brake system has two separate air brake systems that use a single set of controls. If one system loses air, the other system can still provide braking to stop the vehicle. Each system typically operates different brakes, giving redundancy for safety.

In a dual air brake system, before driving you should let air pressure build to at least what level in both systems?

• a.90 psi
• b.50 psi
• c.75 psi
• d.100 psi✓

You should wait until the air pressure builds to at least 100 psi in both the primary and secondary systems before driving. Building to the normal operating range ensures the brakes have enough air to function. Driving with too little pressure is unsafe because braking capacity is reduced.

What is the purpose of the supply pressure gauge in the cab?

• a.To count the number of brake applications
• b.To display engine oil temperature
• c.To measure the vehicle's road speed
• d.To show how much pressure is in the air tanks✓

The supply pressure gauge shows how much air pressure is in the storage tanks. Dual air systems usually have a gauge for each half of the system, or a single gauge with two needles. Watching this gauge helps the driver know the brakes have enough air to operate.

What does the application pressure gauge show?

• a.How much air pressure you are applying to the brakes✓
• b.The total air stored in the reservoirs
• c.The temperature of the brake drums
• d.How many miles remain before service

The application pressure gauge shows how much air pressure you are applying to the brakes when you press the brake pedal. If you notice you must apply more and more pressure to hold your speed on a downgrade, the brakes may be fading. This gauge is not on all vehicles.

What does the brake pedal (foot valve) do in an air brake system?

• a.It engages the parking brake springs
• b.It drains water from the reservoirs
• c.It pumps air into the storage tanks
• d.It controls how much air is sent to apply the brakes✓

Pressing down on the brake pedal, also called the foot valve or treadle valve, sends air to apply the service brakes. Pressing harder applies more air pressure and more braking. Letting up on the pedal reduces the air pressure and releases the brakes.

What is the function of the brake chamber and pushrod?

• a.To store reserve air for the trailer
• b.To convert air pressure into mechanical force that turns the slack adjuster✓
• c.To cool the brake drums during long stops
• d.To warn the driver of low air pressure

When you brake, air enters the brake chamber and pushes the pushrod outward. The pushrod moves the slack adjuster, which rotates the S-cam and applies the brakes. This converts the air pressure into the mechanical force that presses the shoes against the drum.

What is the purpose of the slack adjuster?

• a.To measure the vehicle's stopping distance
• b.To store extra compressed air
• c.To warn the driver when the brakes are hot
• d.To adjust the clearance between the brake shoes and the drum✓

The slack adjuster connects the brake chamber pushrod to the S-cam and adjusts the clearance between the brake linings and the drum. As linings wear, the adjuster keeps them at the proper distance. Manual slack adjusters must be checked and adjusted, while automatic ones self-adjust.

What controls the parking brakes on a vehicle with air brakes?

• a.The trailer hand valve only
• b.The foot brake pedal
• c.A separate parking brake control valve✓
• d.The supply pressure gauge

On air brake vehicles, the parking brakes are set with a separate control valve, often a yellow diamond-shaped knob. Pulling the knob out applies the spring parking brakes, and pushing it in releases them. The parking brakes are held on by spring force, not air pressure.

Why should you NOT use the parking brakes if the brakes are very hot, such as after coming down a steep hill?

• a.It will drain the air tanks
• b.Hot brakes can be damaged, and the drum could contract and crack✓
• c.The knob may be too hot to touch
• d.The parking brakes will not hold on a downgrade

If the brakes are very hot, applying and holding the parking brakes can damage them. On a very hot brake, the drum can contract as it cools around the applied shoes, which may crack the drum. Let the brakes cool before setting the parking brakes when possible.

What is the trailer hand valve (trolley valve) used for?

• a.To release the trailer parking brakes
• b.To adjust the trailer brake slack
• c.To apply only the trailer brakes✓
• d.To drain the trailer air tanks

The trailer hand valve, sometimes called the trolley or Johnson bar, applies only the trailer brakes. It should be used only for testing, not for parking, because trailer air could leak out and release the brakes. Using it for normal braking can also cause the trailer to skid.

Why should the trailer hand valve NOT be used for parking?

• a.It disables the low-pressure warning
• b.Air could leak out and release the trailer brakes✓
• c.It applies too much braking force to the tractor
• d.It drains the primary air tank

You should not use the trailer hand valve for parking because the air could leak out and release the trailer brakes over time. The spring-loaded parking brakes should be used instead, since they are held on by springs and do not depend on air pressure. Relying on the hand valve could let the trailer roll.

What is a one-way check valve used for in an air brake system?

• a.To increase compressor speed
• b.To drain oil from the compressor
• c.To warn the driver of a leak
• d.To keep air from flowing back out of the tanks if pressure drops✓

A one-way check valve allows air to flow in one direction only, keeping air from flowing back out. It is typically placed between the air compressor and the first reservoir. This protects the stored air supply if the compressor or supply side develops a leak.

On many air brake vehicles, what is the purpose of the safety relief valve on the first air tank?

• a.To warn the driver of low pressure
• b.To apply the spring brakes
• c.To automatically drain water
• d.To protect the tank from too much pressure✓

A safety relief valve protects the air tank and the rest of the system from an unsafe buildup of pressure. It is usually set to open at about 150 psi if the governor fails to cut out the compressor. If it releases air, it signals a problem that a mechanic should fix.

Some air brake systems use an alcohol evaporator. What is its purpose?

• a.To help reduce the risk of ice in the brake valves in cold weather✓
• b.To increase the air pressure in the tanks
• c.To warn of low air pressure
• d.To lubricate the brake chambers

An alcohol evaporator puts alcohol into the air system to help reduce the chance of ice forming in the air brake valves and other parts in cold weather. Ice can keep valves from working and cause brake failure. The alcohol container should be checked and filled during cold months.

How are the service brakes applied in an air brake vehicle?

• a.By turning off the engine
• b.By draining the air tanks
• c.By pulling the yellow parking brake knob
• d.By pressing the brake pedal, which sends air to the brake chambers✓

The service brakes are the normal driving brakes and are applied by pressing the brake pedal. Pressing the pedal sends air to the brake chambers, which move the pushrods and apply the brakes. Releasing the pedal lets the air out and releases the brakes.

What is the main advantage of air brakes over hydraulic brakes on large vehicles?

• a.Air is free and the system keeps working even with a small leak by using stored air✓
• b.They make the vehicle stop instantly
• c.They never need any inspection
• d.They eliminate the need for a parking brake

Air brakes are a good way to stop large and heavy vehicles because air is readily available and the brakes use stored compressed air. A small leak will not immediately stop the brakes from working because the system keeps a supply of air. However, air brakes have more parts and take longer to react than hydraulic brakes.

During a walk-around inspection, how do you check manual slack adjusters on S-cam brakes?

• a.By spinning the wheel by hand
• b.By listening for air leaks at the wheel
• c.By measuring the air pressure at each chamber
• d.By pulling hard on each one and checking that it moves no more than about one inch✓

With the parking brakes released and wheels chocked, pull hard on each manual slack adjuster you can reach. If it moves more than about one inch from where the pushrod attaches, it is probably out of adjustment. Brakes that are out of adjustment do not work properly and are unsafe.

When checking the air compressor during inspection, what should you look at if it is belt-driven?

• a.The temperature of the parking brake knob
• b.The reading on the speedometer
• c.The color of the compressed air
• d.The condition and tightness of the compressor drive belt✓

If the air compressor is driven by a belt, you should check the condition and tightness of the belt during your inspection. A loose or worn belt can slip and keep the compressor from building enough pressure. A properly tensioned belt helps the compressor keep the system charged.

Why does an empty truck sometimes take longer to stop than a fully loaded one?

• a.With less weight, the tires have less traction and the brakes can lock more easily, reducing control✓
• b.Empty trucks always use only the trailer brakes
• c.Empty trucks have weaker compressors
• d.Empty trucks have no spring brakes

An empty vehicle has less weight pressing the tires to the road, so the tires have less traction and the wheels can lock up more easily during hard braking. Locked wheels skid and lengthen the stop, so an empty truck can actually require a longer distance to stop under some conditions. Brake smoothly and avoid locking the wheels regardless of load.

What is the correct way to begin the air brake check for build-up rate in a dual air system?

• a.Disconnect the trailer air lines
• b.Drain all the tanks completely first
• c.With the engine at operating rpm, check that pressure builds from about 85 to 100 psi within about 45 seconds✓
• d.Apply the parking brake and rev the engine

In a dual air system, with the engine at operating rpm, the pressure should build from about 85 to 100 psi within about 45 seconds. If air pressure does not build up fast enough, your pressure may drop too low during driving, requiring you to stop and wait to build air. Slow build-up indicates a problem to be fixed.

How do you test the low-pressure warning signal?

• a.By turning the electrical power on and fanning off air until the warning activates✓
• b.By revving the engine to build pressure
• c.By draining the tanks while driving
• d.By pulling on the slack adjusters

With the key on and enough air in the system, step on and off the brake pedal to reduce tank pressure (this is called fanning off the air). The low air pressure warning signal must come on before the pressure drops below 60 psi. If it does not, the warning is faulty and must be repaired.

At what pressure should the spring brakes come on automatically during the continued-air-loss test?

• a.Below 100 psi
• b.Below 90 psi
• c.Below 75 psi
• d.In the range of about 20 to 45 psi✓

Continuing to fan off the air, the spring brakes should come on automatically when tank pressure falls to the manufacturer's specified level, usually in the range of about 20 to 45 psi. On a tractor-trailer, you may hear the parking brake valve pop out. This confirms the automatic emergency braking works.

How do you perform the static leakage (applied pressure) test on a straight truck with air brakes?

• a.With a fully charged system, release the brakes, turn off the engine, apply full brake pressure, and watch that the drop does not exceed the allowed limit✓
• b.Drive at 20 mph and slam on the brakes
• c.Disconnect the trailer and check the gladhands
• d.Drain the tanks and check that they refill on their own

With the air system fully charged (about 125 psi), turn off the engine, release the parking brake, and hold the brake pedal down (fully applied). After the initial pressure drop stabilizes, the air pressure should not drop more than 3 psi in one minute for a straight truck (4 psi for a combination). A greater loss means there is a leak that must be repaired.

For a single vehicle (straight truck), what is the maximum allowable air pressure loss in one minute with the brakes applied?

• a.6 psi
• b.3 psi✓
• c.1 psi
• d.10 psi

For a single vehicle, air pressure should not drop more than 3 psi in one minute when the brakes are fully applied and the engine is off. For a combination vehicle, the limit is 4 psi in one minute. A larger loss indicates a leak too big to be safe.

With the engine off and brakes released, what is the maximum allowable air leakage per minute for a straight truck?

• a.1 psi
• b.2 psi✓
• c.5 psi
• d.3 psi

With the engine off, the air pressure should not drop more than 2 psi in one minute for a single vehicle with the brakes released. For a combination vehicle, the limit is 3 psi per minute. Excessive leakage means the system cannot hold pressure and must be repaired.

How should you test that the service brakes are working before driving?

• a.By pulling on the slack adjusters
• b.By moving the vehicle slowly and firmly applying the brake pedal to feel for a proper stop✓
• c.By draining the air tanks
• d.By listening for the compressor to cut out

After building normal air pressure, move the vehicle forward slowly at about 5 mph and firmly apply the brake pedal. The vehicle should stop and you should not feel any unusual pulling to one side, delay, or unusual feel in the pedal. Any problem should be fixed before driving.

How do you test that the parking (spring) brakes hold?

• a.Drain the tanks and drive forward
• b.Set the parking brake and gently pull forward in low gear to confirm it holds the vehicle✓
• c.Rev the engine with the brakes off
• d.Apply the trailer hand valve only

With the vehicle stopped, apply the parking brake, then gently try to move forward in a low gear against the parking brake. The parking brakes should hold the vehicle in place. If they do not, they are faulty and must be repaired before driving.

What should you check about the brake drums or discs during a walk-around inspection?

• a.That they spin freely by hand
• b.That they are painted a bright color
• c.That they are not cracked and have no loose or missing parts✓
• d.That they contain compressed air

Check that brake drums or discs are not cracked and that there are no loose, missing, or broken parts such as springs. Cracks longer than a certain length can cause brake failure. Also make sure oil or grease is not on the linings, which would reduce braking.

During inspection, brake linings (shoes or pads) should be checked for what?

• a.Correct color and shine
• b.Proper air pressure
• c.Frozen water inside them
• d.Excessive wear and to make sure they are not soaked with oil or grease✓

Brake shoes or pads should not be worn dangerously thin, and they should be free of oil or grease. Linings that are too thin or contaminated cannot provide enough friction to stop the vehicle. Cracked, broken, or missing linings are also out-of-service conditions.

When inspecting the air lines and hoses, what should you look for?

• a.Cracks, cuts, worn spots, and air leaks✓
• b.How fast air moves through them
• c.Their exact length in feet
• d.Bright paint and clean labels

Air lines and hoses should be checked for cracks, cuts, worn spots, and signs of leaks. Damaged lines can burst and cause a loss of air pressure. Listen for hissing that would indicate escaping air.

Why should you drain the air tanks and check for oil or water before a trip if the tanks do not have automatic drains?

• a.To remove moisture and oil that could damage the brakes or freeze✓
• b.To test the parking brake knob
• c.To speed up the compressor
• d.To increase the air pressure

Air tanks collect water and compressor oil, so tanks without automatic drains must be drained manually. Removing this buildup prevents freezing in cold weather and protects brake components. A lot of oil in the tanks can indicate a compressor problem.

During the pre-trip air brake check, when should you check that the pushrod stroke is not too long?

• a.While inspecting the brake chambers and slack adjusters, since too long a stroke means brakes are out of adjustment✓
• b.Only after the trip is finished
• c.Never, because it cannot be checked
• d.Only when the engine is off and cold

When brakes are applied, the pushrod should not travel too far out of the brake chamber. Excessive pushrod stroke means the brakes are out of adjustment and may not stop the vehicle effectively. This is related to checking slack adjusters and is a common out-of-service violation.

What does it mean if the low-pressure warning stays on after you start building air pressure?

• a.The compressor has cut out normally
• b.The brakes are fully adjusted
• c.The parking brake is released
• d.The system pressure is still too low, or the warning device is faulty✓

The low-pressure warning should turn off once air pressure rises above about 60 psi. If it stays on, either the system has not yet built enough pressure or the warning device or gauge is malfunctioning. You should not drive until the cause is found and pressure is normal.

Before the leak-down (static) tests, to what pressure should the air system be brought?

• a.About 20 psi
• b.About 60 psi
• c.About 45 psi
• d.Governor cutout, about 125 psi (fully charged)✓

Before performing the applied and released leak-down tests, the system should be fully charged to about governor cutout pressure (around 125 psi). Starting from full charge gives an accurate measure of how much air leaks out over one minute. Testing from a low pressure would not reveal the true leak rate.

What is the correct order for the brake system portion of a pre-trip inspection regarding air build-up?

• a.Drive the vehicle, then check the compressor
• b.Build pressure to cutout, then test build-up rate, warning signal, spring brake activation, and leak rates✓
• c.Apply the hand valve, then check the tires
• d.Test the parking brake first, then drain all tanks

You first let the system build to governor cutout to confirm the compressor and governor work. Then you check the build-up rate, test the low-pressure warning, continue fanning to confirm the spring brakes apply, and finally check the leak rates. Doing the checks in order confirms each safety feature works.

If you find a lot of oil in the drained air from the tanks, what could this indicate?

• a.The brakes are properly adjusted
• b.The parking brakes are engaged
• c.The tanks are overfilled with air
• d.A possible problem with the air compressor✓

Finding a lot of oil in the air tanks can indicate a problem with the air compressor, such as worn parts allowing oil to pass through. This should be checked by a mechanic. Some oil is normal, but a large amount is a warning sign.

Why is it important to check that the governor cuts the compressor in and out at the correct pressures?

• a.To keep the speedometer accurate
• b.To keep the tires properly inflated
• c.So the system stays within a safe operating pressure range for braking✓
• d.To reduce fuel consumption

The governor must cut out at about 125 psi and cut in at about 100 psi to keep tank pressure in the safe operating range. If it cuts out too high, the safety relief valve may release air; if it cuts in too low, the brakes may not have enough air. Correct settings keep braking reliable.

The total stopping distance for an air brake vehicle includes which of the following?

• a.Only braking distance
• b.Perception distance, reaction distance, brake lag distance, and braking distance✓
• c.Only perception distance
• d.Perception and reaction distance only

For air brakes, total stopping distance is made up of perception distance, reaction distance, brake lag distance, and braking distance. Air brakes have an added brake lag because it takes time for the air to travel through the lines and apply the brakes. This makes the total stopping distance longer than for hydraulic brakes.

What is brake lag in an air brake system?

• a.The time it takes to build air pressure at startup
• b.The distance the vehicle travels before you see a hazard
• c.The time the compressor takes to cut out
• d.The short delay before the brakes work after you press the pedal, because air must travel through the lines✓

Brake lag is the time it takes, after you push the brake pedal, for the air to flow through the lines and actually apply the brakes. On air brakes this adds roughly half a second of travel that hydraulic brakes do not have. Drivers must account for this extra distance when stopping.

What is brake fade?

• a.The brakes slowly releasing while parked
• b.The loss of air pressure overnight
• c.The lag before air reaches the brakes
• d.A reduction in braking power from overheated, overused brakes✓

Brake fade happens when brakes are used so much that they overheat and lose stopping power. Excessive heat causes the drums to expand and the linings to become less effective, so the brakes do not grip as well. Fade is a major risk on long or steep downgrades.

What is the most important thing to do to control your speed on a long, steep downgrade?

• a.Use a low gear so the engine helps hold the vehicle back✓
• b.Pump the brakes continuously as fast as possible
• c.Ride the brakes lightly the whole way down
• d.Shift into neutral and coast

The most important factor on a long downgrade is to select a low gear before starting down, so engine braking helps control your speed. The proper braking technique is then used only to supplement the low gear. Being in the right gear keeps the brakes from overheating and fading.

What is the proper braking technique (snub braking) on a long downgrade?

• a.Never touch the brakes at all
• b.Hold steady light pressure the entire way down
• c.Coast in neutral and brake only at the bottom
• d.Apply the brakes firmly to slow about 5 mph below your safe speed, then release, and repeat✓

With the vehicle in a low gear, apply the brakes firmly enough to feel a definite slowdown, then release when your speed has dropped about 5 mph below your safe speed. Once speed climbs back to the safe speed, repeat the process. This snub-braking method keeps the brakes from overheating compared with light, constant pressure.

On a vehicle with air brakes, when is the proper time to release the brakes during snub braking on a downgrade?

• a.Only when the vehicle comes to a complete stop
• b.When the air pressure warning comes on
• c.As soon as you touch the brake pedal
• d.When your speed has been reduced to about 5 mph below your safe speed✓

During snub braking, you apply the brakes to slow the vehicle and then release them when you have slowed to about 5 mph below your safe speed. This lets the brakes cool between applications. Continuous light braking, by contrast, keeps heat building and can lead to fade.

Compared to a car, how does the stopping distance of a heavy vehicle with air brakes change as weight and speed increase?

• a.It is unaffected by speed
• b.It stays the same regardless of weight
• c.It increases significantly, requiring much greater distance to stop✓
• d.It becomes shorter because of the heavier weight

Heavier vehicles and higher speeds both greatly increase the distance needed to stop. A loaded truck traveling at highway speed needs far more room than a car. Drivers must increase following distance and plan stops well ahead.

If your air brakes fail and pressure drops, what safety feature will eventually stop the vehicle?

• a.The alcohol evaporator
• b.The spring (emergency/parking) brakes applying automatically✓
• c.The trailer hand valve
• d.The service brake pedal alone

If air pressure drops too low, the spring brakes apply automatically because there is no longer enough air to hold the springs back. This is designed to stop the vehicle when the air system fails. However, the driver should react early to a low-pressure warning rather than wait for automatic application.

What should you do first if the low air pressure warning comes on while driving?

• a.Pump the brake pedal rapidly
• b.Brake to a stop safely and get off the road as soon as possible✓
• c.Ignore it until the brakes stop working
• d.Speed up to reach your destination faster

If the low air pressure warning comes on, you should stop and safely park the vehicle as soon as possible because a total loss of air is possible. Continuing to drive risks the spring brakes applying suddenly or the brakes failing. The cause must be found and fixed before continuing.

When stopping normally with a vehicle equipped with air brakes, how should you apply the brakes?

• a.Apply steady, controlled pressure without letting the wheels lock✓
• b.Slam the pedal to the floor immediately
• c.Use only the trailer hand valve
• d.Pump the pedal rapidly the entire stop

For a normal stop, push the brake pedal down with controlled, steady pressure to slow smoothly without locking the wheels. Locked wheels can cause a skid and loss of control, especially on vehicles without anti-lock brakes. Smooth, firm application gives the best control.

Why should you not fan (pump) the brakes on and off on a long downgrade with standard air brakes?

• a.It automatically applies the spring brakes
• b.It makes the brakes too cold
• c.It can use up the air supply faster than the compressor can replace it, reducing braking✓
• d.It speeds up the vehicle

Repeatedly pumping the brakes on and off can use air faster than the compressor can rebuild it, lowering tank pressure. If pressure drops too far, braking is reduced and the spring brakes may apply. Proper technique is to use a low gear and steady snub braking instead.

What is the effect of anti-lock braking systems (ABS) on air brake vehicles?

• a.They help keep the wheels from locking up during hard braking so you can maintain steering control✓
• b.They shorten the vehicle's normal stopping distance dramatically
• c.They replace the need for the parking brake
• d.They eliminate the need to check air pressure

ABS helps prevent the wheels from locking during hard or emergency braking, which helps you keep control and steer. ABS does not necessarily shorten stopping distance or let you drive faster; it is a system to help you retain control. You still brake normally, and the ABS works only when needed.

To make sure you can stop in time, how should you adjust following distance with air brakes at higher speeds?

• a.Follow closely to draft the vehicle ahead
• b.Reduce following distance to save time
• c.Increase following distance because stopping takes longer at higher speeds and heavier weights✓
• d.Keep the same distance as a passenger car

Because heavy vehicles need much more distance to stop, and brake lag adds even more, you should increase your following distance. The faster you go and the heavier you are, the more room you need. Leaving extra space gives you time to react and stop safely.

Before starting down a long grade, when should you select the proper gear?

• a.At the bottom of the hill
• b.Only after the brakes start to fade
• c.Never; leave it in the highest gear
• d.Before starting down, because it is difficult and dangerous to downshift once already descending fast✓

You should select a safe, low gear before you start down a grade. Trying to downshift after you have already gained speed can be difficult or impossible and can leave you without engine braking. Being in the right gear before descending is critical to controlling speed.

What is a proper way to test the trailer service brakes while stopped or moving slowly?

• a.Drain the trailer air tank
• b.Move at about 5 mph and apply the trailer hand valve to confirm the trailer brakes engage✓
• c.Set the parking brake and rev the engine
• d.Slam the tractor foot brake to the floor

To test the trailer service brakes, move the vehicle slowly (about 5 mph) and apply the trailer hand valve. You should feel the brakes come on, which confirms the trailer brakes and their air connection are working. This should be done as part of your pre-trip checks, not used for normal stopping.