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.