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Section 5: Air Brakes

If you want to drive a truck or bus with air brakes or pull a trailer with air brakes, you need to study this section. If you want to pull a trailer with air brakes you also need to study Section 6: Combination Vehicles.

Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles when the brakes are well maintained and used properly.

Air brake systems combine three braking systems: service, parking and emergency brake systems.

  • The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.

5.1 Parts of Air Brake System

There are many parts to an air brake system. You should know about the parts discussed here.

Air Compressor

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

Air Compressor Governor

The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the “cut-out” level (around 125 pounds per square inch, or “psi”), the governor stops the compressor from pumping air. When the tank pressure falls to the “cut-in” pressure (around 100 psi), the governor allows the compressor to start pumping again.

Air Storage Tanks

Air storage tanks are used to hold compressed air. The number and size of air tanks vary among vehicles. The tanks will hold enough air to allow the brakes to be used several times even if the compressor stops working.

Air Tank Drains

Compressed air usually has some water and compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure to drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types of drain valves:

  • Manually operated by turning a quarter turn (see Figure 5-1) or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
  • Automatic, in which the water and oil are automatically expelled. They may be equipped for manual draining as well.

The automatic types are available with electric heating devices. These help prevent freeze-up of the automatic drain in cold weather

Figure 5-1
Manual Drain Valve

Alcohol Evaporator

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).

Safety Valve

A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault repaired by a mechanic.

Make sure you study and memorize all of the below definitions. You must know how to distinguish between the three different types of brakes for the written exam. You will see these terms many times so it's very important to memorize them now.
You'll need to study and know each part listed in section 5.1 for the air brake system. Use flash cards if you need to, but all of these parts must be memorized.
For the written exam, you'll need to understand the difference between "cut out level" and "cut in pressure". You'll also need to know the air pressure level for each. Cut out pressure is around 125psi and cut in pressure is around 100psi. Be sure you have this memorized as it will come up quite often during your training.
Remember: Manual air tank drains must be drained daily in order to get rid of water and oil inside the air tanks.
You may be asked about this on the written exam. Make sure you remember that the safety relief valve generally opens at 150 psi.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

Review Questions - Click On The Picture To Begin...

Which of the following statements is true about Air Tank Drains?
  • All fluids from all air tanks drain into a central location which must be drained
  • Manually operated air tank drains normally use a handle or a cable to drain the system
  • Automatic air tank drains always have to be drained manually as well
  • Air tank drains should be forced into the open position during dry weather so that they drain throughout the day

Quote From The CDL Manual:

Air Tank Drains - Compressed air usually has some water and compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure to drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types of drain valves:

  • 1. Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
  • 2. Automatic, in which the water and oil are automatically expelled. They may be equipped for manual draining as well.

The automatic types are available with electric heating devices. These help prevent freeze-up of the automatic drain in cold weather.

Next
How often should you drain air tanks with manual air tank drains?
  • Once per month
  • Once per week
  • Once per day
  • Every 3 months

Quote From The CDL Manual:

You must drain the tanks yourself at the end of each day of driving.

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Next
What does it mean for the air compressor to "cut in"?
  • The air compressor will begin supplying air to the air storage tanks
  • The air compressor will begin releasing air from the air storage tanks
  • The air compressor will stop releasing air from the air storage tanks
  • The air compressor will stop supplying air to the air storage tanks

Quote From The CDL Manual:

Air Compressor - The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

Air Compressor Governor - The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level (around 125 pounds per square inch, or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.

TruckingTruth's Advice:

Be sure to understand how the air compressor and air compressor governor both work. The air compressor governor does not supply air, rather, it tells the air compressor when it should or shouldn't be supplying air to the air storage tanks.

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What does the Air Compressor Governor do?
  • Controls when the air compressor will pump air into the air storage tanks
  • Ensures that air never stops flowing into the air storage tanks
  • Activates the emergency brakes when air pressure gets too high
  • Detects any leaks in the airlines

Quote From The CDL Manual:

The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level (around 125 pounds per square inch, or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.

Prev
Next
Why do air tanks need to be manually drained?
  • Draining the air tanks helps to ensure the air pressure isn't building too high in the tanks
  • Compressed air causes water to form and condense which is bad for the air brake system, especially if it freezes during cold weather
  • The brake system sends water through the brake lines to flush the air lines out and the water needs to be drained through the air tanks before another system flush can occur
  • Draining air tanks helps bring the air pressure lower which can then be used to test the air governor

Quote From The CDL Manual:

Air Tank Drains - Compressed air usually has some water and compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure to drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types of drain valves:

  • 1. Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
  • 2. Automatic, in which the water and oil are automatically expelled. They may be equipped for manual draining as well.

The automatic types are available with electric heating devices. These help prevent freeze-up of the automatic drain in cold weather.

Prev
Next
What are Air Storage Tanks?
  • Air storage tanks are used to hold compressed air
  • Air storage tanks transfer air from the tractor to the trailer on combination vehicles
  • Air storage tanks are used to keep the parking brakes active while stopped
  • Air storage tanks release excess air pressure in the brake system

Quote From The CDL Manual:

Air storage tanks are used to hold compressed air. The number and size of air tanks vary among vehicles. The tanks will hold enough air to allow the brakes to be used several times even if the compressor stops working.

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Next
What is the purpose of Air Storage Tanks?
  • They are used to supply air to the emergency brakes in the event of an air compressor failure
  • They are used to supply air to the air compressor
  • They are used to transport air from the tractor to the trailer on combination vehicles
  • They are used to hold compressed air for the air brake system

Quote From The CDL Manual:

Air storage tanks are used to hold compressed air. The number and size of air tanks vary among vehicles. The tanks will hold enough air to allow the brakes to be used several times even if the compressor stops working.

TruckingTruth's Advice:

In an air brake system, emergency brakes naturally want to activate. It is the air pressure which holds the emergency brakes back and keeps them from activating. Therefor, air is not required for emergency brakes to function. Rather, if air is depleted from the air tanks, the emergency brakes will automatically engage.

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What is the service brake?
  • A system which applies and releases the parking brakes when you use the parking brake control
  • A system used by mechanics during servicing do deactivate the emergency brakes
  • A system which uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.
  • A system which applies and releases the brakes when you use the brake pedal during normal driving

Quote From The CDL Manual:

Air brake systems combine three braking systems: service, parking and emergency brake systems.

  • 1. The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • 2. The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • 3. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.

TruckingTruth's Advice:

It is vital that you learn the difference between the following definitions:

  • Service Brake
  • Parking Brake
  • Emergency Brake

Memorize each definition and be sure you understand how each of these differ. Not only will this be critically important to passing the air brakes endorsement test but it is terminology you'll need to understand in the real world.

Prev
Next
Depressing the brake pedal activates which brake system?
  • The service brakes
  • The parking brakes
  • The driving brakes
  • The emergency brakes

Quote From The CDL Manual:

Air brake systems combine three braking systems: service, parking and emergency brake systems.

  • 1. The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
  • 2. The parking brake system applies and releases the parking brakes when you use the parking brake control.
  • 3. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.

TruckingTruth's Advice:

It is very important that you memorize the difference between the service brake, parking brake, and emergency brake. Study what the function is of each as you'll likely be asked about all 3 on the written exam. These are also terms you will need to know when out in the "real world."

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Next
The air compressor governor will begin supplying air around what PSI?
  • Around 75 psi
  • Around 50 psi
  • Around 125 psi
  • Around 100 psi

Quote From The CDL Manual:

When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.

TruckingTruth's Advice:

Make sure you memorize the air pressure for when the governor will "cut in" and "cut out" as you will need to know this for the written exam, the pre-trip exam, and in the real world while checking your gauges going down the road.

Prev
Next
At what pressure will the air compressor governor tell the air compressor to "cut out"?
  • Around 75 psi
  • Around 150 psi
  • Around 100 psi
  • Around 125 psi

Quote From The CDL Manual:

When air tank pressure rises to the "cut-out" level (around 125 pounds per square inch, or "psi"), the governor stops the compressor from pumping air.

TruckingTruth's Advice:

Be sure to understand that the air compressor governor has one main purpose and that is to tell the air compressor when to pump air and when to stop pumping air. The air compressor governor will tell the air compressor to start supplying air (cut in) and when to stop supplying air (cut out).

  • Cut in air pressure is usually when air pressure falls to about 100 psi.
  • Cut out air pressure is usually when air pressure raises to about 125 psi
Prev
Next
Which of the following is true about the Air Compressor?
  • It does not connect to the main engine and runs off of a separate independent motor
  • It has a separate fuel tank which must be filled every 100 to 200 miles
  • It sucks air out of the air storage tanks (reservoirs)
  • It may have its own oil supply or be lubricated by engine oil

Quote From The CDL Manual:

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

Prev
Next
What is the purpose of Air Tank Drains?
  • They allow air to be released when the air pressure builds too high
  • They allow oil to be inserted into the air tanks
  • They allow water to be released from the air tanks
  • They are used to transfer air from the air tank to the brake chambers

Quote From The CDL Manual:

Air Tank Drains - Compressed air usually has some water and compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure to drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types of drain valves:

  • 1. Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.
  • 2. Automatic, in which the water and oil are automatically expelled. They may be equipped for manual draining as well.

The automatic types are available with electric heating devices. These help prevent freeze-up of the automatic drain in cold weather.

Prev
Next
What does an Alcohol Evaporator do?
  • Helps reduce the risk of ice in air brake valves and other parts during cold weather
  • Supplies an alcohol mixture to the brake pads, keeping them clean from foreign debris
  • Keeps the air lines free from sand, dirt, and dust
  • Injects alcohol into the air compressor, keeping moving parts clean and free from debris

Quote From The CDL Manual:

Alcohol Evaporator - Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).

Prev
Next
Which of the following statements about the Air Compressor is false?
  • Air compressors may have their own oil supply or be lubricated by engine oil
  • Air compressors are always air cooled
  • The air compressor pumps air into the air storage tanks
  • The air compressor is connected to the engine through gears or a V-belt

Quote From The CDL Manual:

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

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