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Ontario Air Brake (Z) overview

Ontario Air Brake (Z): Air supply subsystem

Compressor, governor cut-in and cut-out, the 80 to 145 psi operating range, wet and dry tanks, draining, air dryers, alcohol evaporators, safety valves and low-air warnings.

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Air supply subsystem · Question 1Score 0/0

How often do many manufacturers recommend draining air tanks?

Study reference: MTO Air Brake Handbook (online edition, Feb. 2026), Ch. 2 Air Supply Subsystem — Compressor, governor, tanks, warnings

All 31 questions in this test

Every question and its answer choices. The correct answers and explanations appear when you take the test above.

  1. How often do many manufacturers recommend draining air tanks?

    1. Weekly
    2. At each annual inspection
    3. Monthly
    4. Daily

  2. Why does water collect at the bottom of air tanks?

    1. The air dryer adds water vapour to it
    2. Brake fluid separates in cold weather
    3. Rain gets in through open gladhands
    4. Moisture condenses as air is compressed

  3. Which problem can moisture and oil sludge from the wet tank cause?

    1. Faster air build-up after each stop
    2. Shorter pushrod stroke at every wheel
    3. Damaged seals and sticking valves
    4. Higher cut-out pressure on the governor

  4. In cold weather, why is moisture in the air system dangerous?

    1. It can freeze and cause brake failure
    2. It thickens the brake fluid
    3. It makes the spring brakes release
    4. It raises the governor setting

  5. What is the lowest actual cut-in pressure allowed?

    1. 100 psi (690 kPa)
    2. 55 psi (380 kPa)
    3. 60 psi (414 kPa)
    4. 80 psi (552 kPa)

  6. Why are the primary and secondary tanks called dry tanks?

    1. They hold only alcohol vapour
    2. They are drained by the air dryer
    3. They gather less moisture
    4. They contain no air until braking

  7. What is a wig wag?

    1. A valve that drains the wet tank
    2. A knob that releases the trailer brakes
    3. An arm that drops when air is low
    4. A gauge that shows pushrod stroke

  8. Why must compressor brackets and fasteners be kept secure?

    1. So it keeps supplying air reliably
    2. So the governor can cut out much sooner
    3. So the tanks always stay below 80 psi
    4. So the drain valves stay closed while driving

  9. Your truck has an air dryer. Do you still need to drain the tanks?

    1. No, the dryer removes all moisture
    2. Yes, still drain them regularly
    3. Only the secondary tank needs it
    4. Only during the winter months

  10. Why are there primary and secondary air tanks and circuits?

    1. So one circuit can stop you if one fails
    2. To double the air pressure available
    3. To feed the trailer and tractor separately
    4. So the compressor can rest between stops

  11. Which tank should you always drain first?

    1. The primary tank
    2. The secondary tank
    3. The supply (wet) tank
    4. The tank nearest the cab

  12. When does an air dryer expel the moisture it has collected?

    1. When the compressor reaches cut-out
    2. Each time the pedal is released
    3. Only when the engine is shut off
    4. When the low-air warning activates

  13. What does an alcohol evaporator do?

    1. Helps stop moisture from freezing
    2. Cleans oil off the brake linings
    3. Cools the compressor on hot days
    4. Lubricates the slack adjusters daily

  14. Where is an air dryer located?

    1. Between the compressor and the supply tank
    2. Between the foot valve and brake chambers
    3. Inside the secondary tank
    4. On the trailer supply line

  15. You hear the supply tank safety valve venting air. What does it mean?

    1. The compressor has just reached cut-in
    2. The tanks are being drained normally
    3. Pressure is too high; repair it now
    4. The spring brakes are releasing now

  16. Which tank does compressed air enter first?

    1. The trailer reservoir
    2. The primary tank
    3. The supply (wet) tank
    4. The secondary tank

  17. At about what pressure does a safety valve normally open?

    1. 120 psi (828 kPa)
    2. 100 psi (690 kPa)
    3. 200 psi (1380 kPa)
    4. 150 psi (1035 kPa)

  18. What is the highest actual cut-out pressure allowed?

    1. 120 psi (828 kPa)
    2. 145 psi (1000 kPa)
    3. 175 psi (1207 kPa)
    4. 150 psi (1035 kPa)

  19. What powers the air compressor on a truck with air brakes?

    1. Pressure stored in the spring brakes
    2. A separate electric motor
    3. The vehicle's engine
    4. The rotation of the trailer wheels

  20. The low-air warning must activate before pressure in either tank falls to what level?

    1. 20 psi (138 kPa)
    2. 45 psi (311 kPa)
    3. 55 psi (380 kPa)
    4. 80 psi (552 kPa)

  21. What is the cut-in pressure?

    1. Where the safety valve opens to vent air
    2. Where the spring brakes begin to apply
    3. Where the low-air warning device comes on
    4. Where the compressor resumes pumping

  22. What stops air from flowing back into the supply tank?

    1. One-way check valves
    2. The drain valve
    3. The safety valve
    4. The governor

  23. What must a low air-pressure warning device always include?

    1. A visual warning such as a light
    2. A loud horn outside the cab
    3. An automatic engine shutdown
    4. A dropping arm on every truck

  24. Your gauge shows an unusual pressure reading. Who should confirm it?

    1. No one; dash gauges are exact
    2. A technician using an accurate gauge
    3. The next driver of the vehicle
    4. A DriveTest examiner

  25. What do the dash air pressure gauges show?

    1. Pressure inside each brake chamber
    2. Pushrod stroke at each of the wheels
    3. Air temperature inside the compressor
    4. Primary and secondary tank pressure

  26. How far above cut-in pressure is cut-out pressure normally set?

    1. 5 to 10 psi higher
    2. 40 to 50 psi higher
    3. 60 to 70 psi higher
    4. 20 to 25 psi higher

  27. Why should you stay out of the path of air exhausting from brake valves?

    1. Its debris can hurt eyes and ears
    2. It is cold enough to cause frostbite
    3. It contains alcohol vapour that can ignite
    4. It can reverse the one-way check valves

  28. What part makes the compressor stop and start pumping air at set pressures?

    1. The safety valve
    2. The governor
    3. The foot valve
    4. The check valve

  29. How does an air compressor raise the pressure of air?

    1. By mixing it with alcohol vapour first
    2. By squeezing it into a smaller space
    3. By heating it inside the supply tank
    4. By cooling it as it passes the air dryer

  30. What is a typical normal operating range for an air brake system?

    1. About 60 to 80 psi
    2. About 150 to 170 psi
    3. About 40 to 60 psi
    4. About 100 to 120 psi

  31. While driving, pressure falls below the normal cut-in setting. What should you do?

    1. Drive on until the warning sounds
    2. Stop safely as soon as possible
    3. Pump the pedal to rebuild pressure
    4. Rev the engine and keep going

Air supply subsystem: key rules to remember

Our summary of what the official handbook says on this topic, in our own words. Read it before or after the practice questions above, then check the original chapter. Read the official chapter ↗

Compressor and governor

  • The air supply subsystem produces, stores and manages the compressed air the brakes use.
  • The engine-driven air compressor squeezes air into a smaller space, raising its pressure; compressed air is stored energy.
  • Compressors are driven directly by the engine or by belts and pulleys, and mounted on the engine or with brackets and fasteners that must stay secure.
  • The compressor turns whenever the engine runs; the governor tells it when to stop pumping (cut-out) and when to start again (cut-in).
  • Pressure is measured in kilopascals (kPa) or pounds per square inch (psi).

Normal pressure range

  • Cut-out is normally 20 to 25 psi (138–173 kPa) above cut-in.
  • A typical operating range is about 100 to 120 psi (690–828 kPa).
  • Actual cut-out must never be above 145 psi (1000 kPa); actual cut-in must never be below 80 psi (552 kPa).
  • Drive only when pressure is in the normal range; a drop below the normal cut-in point signals a malfunction or abnormal demand, so stop safely as soon as possible.
  • Abnormal readings should be confirmed by a technician with an accurate gauge.

Air tanks

  • Tanks are usually steel cylinders with domed ends under or around the frame; a vehicle may have one, two, three or more, and two tanks can share one shell with an internal divider.
  • Moisture condenses as air is compressed, and compressor oil can mix in; both settle at the bottom of the tanks.
  • The first tank, the supply or "wet" tank, collects most of the moisture and oil.
  • Sludge from moisture and oil can damage seals and make valves stick; the mixture can corrode tanks; moisture can freeze and cause brake failure; buildup reduces air volume.
  • One-way check valves stop air flowing back into the supply tank.
  • From the supply tank air splits into primary and secondary (dual-service, "dry") tanks; if one circuit fails, the other can still stop the vehicle.

Draining

  • Many manufacturers recommend draining the tanks daily through the drain valve at the bottom of each tank.
  • Always drain the supply (wet) tank first, so its moisture cannot flow into the next tank.
  • Check that each tank drains fully and watch the discharge for anything abnormal.
  • Vehicles with air dryers still need their tanks drained regularly.

Air dryer and alcohol evaporator

  • An air dryer sits between the compressor and the supply tank; air cools and passes through a drying material there.
  • The dryer expels the collected moisture with a burst of air when the compressor reaches cut-out.
  • An alcohol evaporator adds alcohol vapour to the air to lower the chance of moisture freezing; only products made for it may be used.

Gauges, safety valve, warnings

  • Every air-braked vehicle must have working air gauges on the dash, either two gauges (primary and secondary) or one gauge with two needles, in kPa or psi.
  • Safety valves on the supply tank and air dryer normally open at 150 psi (1035 kPa) if the governor fails; a venting safety valve needs immediate repair.
  • The low air-pressure warning must come on before pressure in either tank falls to 55 psi (380 kPa); most activate at 60 psi (414 kPa) or higher.
  • The warning must be visual (a light) and may also sound a buzzer, bell or alarm; a "wig wag" arm drops into the driver's view on some vehicles.
  • A low-air warning means a serious brake hazard: stop safely as soon as possible.

Compressed air hazards

  • Exhausting air moves fast and carries moisture, oil, dirt and dust that can hurt eyes and hearing.
  • Air exhausts at brake valve ports, near the axles, at the air dryer exhaust and at tank drain valves.
  • Never dismantle, remove, repair or tamper with any brake component.

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