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Air brake parts: making, storing and using the air

Air Brakes7 minManual section 5, 5.1.1–5.1.8

Air brakes stop a heavy truck or bus with compressed air. To pass the Air Brakes test you must know each part, what it does, and the key pressure numbers. This lesson follows the air from the compressor to the brakes at the wheels.

Three brake systems in one

Air brakes use compressed air to make the brakes work. They are a good, safe way to stop large and heavy vehicles, but only if they are well maintained and used the right way.

An air brake setup is really three different braking systems:

  • Service brake system: puts on and releases the brakes when you use the brake pedal in normal driving.
  • Parking brake system: sets and frees the parking brakes when you work the parking brake control in the cab.
  • Emergency brake system: borrows parts of the service and parking brake systems to bring the vehicle to a stop when the brake system fails.

You need this section to drive a truck or bus with air brakes, or to pull a trailer with air brakes. If you pull a trailer, you must also study the Combination Vehicles section.

Making and storing the air: compressor, governor, tanks

The Air compressor pumps air into the Air storage tanks (also called reservoirs). The engine drives the compressor through gears or a v-belt.

The compressor may be air cooled or cooled by the engine cooling system. It may use engine oil or have its own oil supply. If it has its own oil supply, check that oil level before you drive.

The Governor (air compressor governor) controls when the compressor pumps air into the tanks:

  • Cut-out pressure, about 125 psi: when tank pressure rises to this level, the governor stops the compressor from pumping.
  • Cut-in pressure, about 100 psi: when tank pressure falls to this level, the governor lets the compressor start pumping again.

The air tanks hold the compressed air. The number and size of tanks is different on different vehicles. The tanks hold enough air to use the brakes several times, even if the compressor stops working. That reserve is why you can still stop safely right after a compressor failure.

Keeping water and oil out: tank drains and alcohol evaporator

Compressed air usually carries some water and some compressor oil. Both harm the brake system. For example, water can turn to ice in cold weather, and then the brakes can fail. Water and oil settle at the bottom of each tank, which is why every tank has a drain valve at the bottom. Drain the tanks completely.

  • Manual drain: you open it by turning it a quarter turn or by pulling a cable. You must drain these tanks yourself at the end of each day of driving.
  • Automatic drain: pushes out water and oil by itself. These tanks may also be set up for manual draining. Automatic drains can come with electric heating devices that keep the drain from freezing in cold weather.

Some systems have an Alcohol evaporator. It adds alcohol to the air so ice is less likely to form in brake valves and other parts in cold weather. If ice forms inside the system, the brakes can fail.

During cold weather, check the alcohol container every day and fill it as needed. The alcohol does not replace draining: you still must drain the tanks daily to remove water and oil, unless the system has automatic drain valves.

Safety valve and brake pedal

A Safety valve (safety relief valve) sits in the first tank that the compressor fills. Its job is to keep too much pressure from damaging that tank and the rest of the system. It is usually set to open at 150 psi.

If the safety valve lets air out, something is wrong (for example, the governor is not cutting out). Have a mechanic fix the fault.

You put on the brakes by pushing the brake pedal, also called the Foot valve or Treadle valve. Pushing harder applies more air pressure. Letting up reduces the pressure and releases the brakes.

Every time you release the brakes, some compressed air leaves the system, and the compressor must make it up. If you press and release the pedal when you do not need to (fanning or pumping), air can leave faster than the compressor can replace it. If pressure gets too low, the brakes will not work. So with air brakes: do not pump the pedal.

Foundation brakes at the wheels

Foundation brakes are the brakes at each wheel. The type you will see most often is the S-cam drum brake.

Drums, shoes and linings. There is a brake drum at each end of the axles, and the wheels are bolted to the drums. The braking parts sit inside the drum. When you brake, the brake shoes and linings press against the inner surface of the drum. This makes friction, which slows the vehicle and also makes heat. How much heat a drum can handle without damage depends on how hard and how long you brake. Too much heat can make the brakes stop working.

How an S-cam brake works, step by step:

  • You push the brake pedal, and air goes into each Brake chamber.
  • Air pressure pushes the push rod out. The push rod moves the Slack adjuster, which twists the brake camshaft.
  • The camshaft turns the S-cam (named for its "S" shape). The S-cam forces the brake shoes apart and presses them against the inside of the drum.
  • When you release the pedal, the S-cam turns back and a spring pulls the shoes away from the drum, so the wheels roll freely again.

Wedge brakes. The push rod from the brake chamber drives a wedge straight in between the ends of two brake shoes, forcing them out against the drum. There may be one brake chamber or two (with wedges pushed in at both ends of the shoes). Wedge brakes may be self-adjusting or may need manual adjustment.

Disc brakes. In air disc brakes, air works on a brake chamber and slack adjuster just as in S-cam brakes. But instead of an S-cam there is a power screw. The force of the chamber on the slack adjuster turns the power screw, which clamps the disc (rotor) between the lining pads of a caliper, like a large C-clamp.

You will see wedge brakes and disc brakes less often than S-cam brakes.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 5, 5.1.1–5.1.8

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. Air brakes are really three braking systems. Which list is correct?
    1. Service, hydraulic and parking
    2. Primary, secondary and trailer
    3. Service, parking and emergency
    4. Foot, hand and engine
    Show answer
    Service, parking and emergency

    An air brake setup is made of the service brake, parking brake and emergency brake systems.

  2. The emergency brake system works by:
    1. Using parts of the service and parking brake systems
    2. Using a separate set of hydraulic brakes
    3. Using only the engine retarder
    4. Using only the trailer brakes
    Show answer
    Using parts of the service and parking brake systems

    The emergency system uses parts of the service and parking systems to stop the vehicle if the brake system fails.

  3. What does the air compressor do?
    1. Controls when air is pumped into the tanks
    2. Pumps air into the air storage tanks
    3. Releases air when pressure is too high
    4. Shows the driver the tank pressure
    Show answer
    Pumps air into the air storage tanks

    The compressor pumps air into the storage tanks (reservoirs). The governor decides when it pumps.

  4. The air compressor is connected to the engine through:
    1. The transmission
    2. The fuel pump
    3. The alternator only
    4. Gears or a v-belt
    Show answer
    Gears or a v-belt

    The engine drives the compressor through gears or a v-belt.

  5. If the air compressor has its own oil supply, you should:
    1. Check the oil level before driving
    2. Drain the oil every day
    3. Add alcohol to the oil in winter
    4. Ignore it; it never needs checking
    Show answer
    Check the oil level before driving

    A compressor with its own oil supply needs its oil level checked before you drive.

  6. The air compressor governor controls:
    1. How hard the brakes are applied
    2. The speed of the engine
    3. When the compressor pumps air into the tanks
    4. When the spring brakes come on
    Show answer
    When the compressor pumps air into the tanks

    The governor decides when the compressor starts and stops pumping air into the tanks.

  7. At about what tank pressure does the governor stop the compressor from pumping (cut-out)?
    1. 100 psi
    2. 125 psi
    3. 150 psi
    4. 85 psi
    Show answer
    125 psi

    Cut-out is around 125 psi. Cut-in is around 100 psi, and the safety valve opens around 150 psi.

  8. The governor lets the compressor start pumping again (cut-in) when tank pressure falls to about:
    1. 100 psi
    2. 55 psi
    3. 125 psi
    4. 60 psi
    Show answer
    100 psi

    Cut-in pressure is about 100 psi.

  9. If the air compressor stops working, the air storage tanks:
    1. Empty right away
    2. Lock all the wheels at once
    3. Switch the brakes to hydraulic
    4. Hold enough air to use the brakes several times
    Show answer
    Hold enough air to use the brakes several times

    The tanks store enough air to use the brakes several times even if the compressor fails.

  10. Why must air tanks be drained?
    1. To lower the pressure for the governor test
    2. Water and compressor oil collect in them and can cause brake failure
    3. To let the alcohol evaporator refill
    4. To cool the compressor
    Show answer
    Water and compressor oil collect in them and can cause brake failure

    Water and oil collect at the bottom of the tanks. Water can freeze and cause brake failure.

  11. A manually operated air tank drain is opened by:
    1. Pressing the brake pedal three times
    2. Pulling the yellow parking knob
    3. Turning it a quarter turn or pulling a cable
    4. Running the engine at fast idle
    Show answer
    Turning it a quarter turn or pulling a cable

    Manual drains open with a quarter turn or by pulling a cable.

  12. How often must you drain air tanks that have manual drains?
    1. At the end of each day of driving
    2. Once a week
    3. Only in cold weather
    4. Only when the low-air warning comes on
    Show answer
    At the end of each day of driving

    Tanks with manual drains must be drained by the driver at the end of each day of driving.

  13. Some automatic air tank drains have electric heating devices. What are they for?
    1. To warm the cab
    2. To dry the brake linings
    3. To raise air pressure faster
    4. To keep the automatic drain from freezing in cold weather
    Show answer
    To keep the automatic drain from freezing in cold weather

    Heaters help prevent the automatic drain from freezing in cold weather.

  14. The alcohol evaporator puts alcohol into the air system to:
    1. Remove oil from the tanks
    2. Reduce the risk of ice in valves and other parts in cold weather
    3. Lubricate the compressor
    4. Cool the brake drums
    Show answer
    Reduce the risk of ice in valves and other parts in cold weather

    Alcohol lowers the risk of ice forming in brake valves and other parts in cold weather.

  15. True or False: If your vehicle has an alcohol evaporator, you no longer need to drain the air tanks each day.
    1. True
    2. False
    Show answer
    False

    False. You still must drain water and oil daily unless the system has automatic drains.

  16. During cold weather, how often should you check and fill the alcohol container?
    1. Once a month
    2. Only before long trips
    3. Every day
    4. Only when the brakes freeze
    Show answer
    Every day

    In cold weather, check the alcohol container and fill it as needed every day.

  17. The safety valve is usually set to open at:
    1. 100 psi
    2. 125 psi
    3. 55 psi
    4. 150 psi
    Show answer
    150 psi

    The safety relief valve, in the first tank the compressor fills, usually opens at 150 psi.

  18. If the safety valve releases air, what should you do?
    1. Have a mechanic fix the problem; something is wrong
    2. Nothing; it is normal every few minutes
    3. Pump the brake pedal to lower pressure
    4. Close the valve by hand and keep driving
    Show answer
    Have a mechanic fix the problem; something is wrong

    A safety valve letting out air means something is wrong, so have a mechanic fix it.

  19. Pressing and releasing the brake pedal when it is not needed can:
    1. Raise the air pressure
    2. Adjust the slack adjusters
    3. Let air out faster than the compressor can replace it
    4. Cool the brakes
    Show answer
    Let air out faster than the compressor can replace it

    Each release lets air out. Unneeded pumping can drain air faster than the compressor makes it, and the brakes may not work.

  20. The brake pedal in an air brake system is also called the:
    1. Relay valve
    2. Foot valve or treadle valve
    3. Quick release valve
    4. Glad hand
    Show answer
    Foot valve or treadle valve

    The brake pedal is also known as the foot valve or treadle valve.

  21. What is the most common type of foundation brake on trucks?
    1. S-cam drum brake
    2. Wedge brake
    3. Air disc brake
    4. Band brake
    Show answer
    S-cam drum brake

    The s-cam drum brake is the most common. Wedge and disc brakes are less common.

  22. In an S-cam brake, what does the push rod move when air enters the brake chamber?
    1. The brake drum
    2. The governor
    3. The wheel bearing
    4. The slack adjuster, which twists the brake camshaft
    Show answer
    The slack adjuster, which twists the brake camshaft

    Air pushes the rod out, moving the slack adjuster, which twists the camshaft and turns the S-cam.

  23. When you release the brake pedal on an S-cam brake, what pulls the brake shoes away from the drum?
    1. Air pressure
    2. The slack adjuster
    3. A spring
    4. The power screw
    Show answer
    A spring

    The S-cam turns back and a spring pulls the shoes away from the drum so the wheel rolls freely.

  24. In air-operated disc brakes, what takes the place of the S-cam?
    1. A wedge
    2. A power screw
    3. A brake drum
    4. A hydraulic cylinder
    Show answer
    A power screw

    A power screw, turned by the chamber and slack adjuster, clamps the rotor between caliper pads.

  25. What happens when brakes are used too hard or too long and the drums get too hot?
    1. The brakes can stop working
    2. The brakes grip better
    3. The compressor shuts off
    4. The ABS turns on
    Show answer
    The brakes can stop working

    Braking makes heat. Too much heat can make the brakes stop working.

  26. Which statement about wedge brakes is correct?
    1. They are the most common air brake
    2. They use a power screw and caliper
    3. They can never be adjusted
    4. They may have one or two brake chambers and may be self-adjusting or need manual adjustment
    Show answer
    They may have one or two brake chambers and may be self-adjusting or need manual adjustment

    Wedge brakes push a wedge between the shoe ends, may have one or two chambers, and may self-adjust or need manual adjustment.