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Ontario Air Brake (Z) overview

Ontario Air Brake (Z): Trailer brakes

Gladhands and colour coding, cross-connected lines, the trailer hand valve, the red trailer supply valve, the tractor protection valve and spring or service brake priority.

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Trailer brakes · Question 1Score 0/0

What controls the tractor protection valve?

Study reference: MTO Air Brake Handbook (online edition, Feb. 2026), Ch. 5 Trailer Brake Subsystem — Couplers, supply valve, tractor protection valve

All 24 questions in this test

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

  1. What controls the tractor protection valve?

    1. The foot valve only
    2. The governor
    3. The spring brake control valve
    4. The trailer supply valve

  2. What does the trailer supply valve knob normally look like?

    1. Blue and round
    2. Red and eight-sided
    3. Yellow and four-sided
    4. Black and triangular

  3. Why does the trailer supply valve usually close before the dash gauges fall to 20–45 psi?

    1. Line pressure falls faster than tanks
    2. The drain valves open before it does
    3. The gauges read the trailer tanks only
    4. The governor closes it automatically first

  4. What are the metal trailer air line couplers commonly called?

    1. Glad valves
    2. Kingpins
    3. Gladhands
    4. Clevis pins

  5. What can happen if the trailer supply and service lines are cross-connected?

    1. Spring brakes may stay on; air is lost
    2. Only the trailer ABS light comes on
    3. The trailer brakes work twice as hard
    4. Nothing; the valves correct it automatically

  6. How are two gladhands joined?

    1. Screwed together clockwise by hand
    2. Seals together at an angle, then rotated
    3. Pushed straight together until they click
    4. Clamped together with a metal hose clip

  7. How do you supply air to a trailer with the trailer supply valve?

    1. Hold it for five seconds
    2. Push the knob in
    3. Turn it to the left
    4. Pull the knob out

  8. Below what supply-line pressure must trailer spring brakes apply automatically?

    1. 60 psi (414 kPa)
    2. 120 psi (828 kPa)
    3. 20 psi (138 kPa)
    4. 90 psi (621 kPa)

  9. What is the usual colour code for trailer air couplers?

    1. Red for service, black for supply
    2. Yellow for supply, green for service
    3. Blue for supply, red for service
    4. Red for supply, blue for service

  10. How does the brake system of a converter dolly compare with a trailer's?

    1. It has no brakes at all
    2. It has only spring brakes
    3. It uses hydraulic brakes only
    4. It is essentially the same

  11. What does the trailer hand valve do?

    1. Applies only the trailer service brakes
    2. Releases the tractor's spring brakes
    3. Closes the tractor protection valve
    4. Fills the trailer air tanks quickly

  12. If the trailer breaks away, at what supply-line pressure does the trailer supply valve close by itself?

    1. Between 60 and 80 psi
    2. Only at 0 psi
    3. Between 20 and 45 psi
    4. Between 100 and 120 psi

  13. What is the job of the tractor protection valve?

    1. Raise air pressure in the trailer
    2. Limit braking on the steering axle
    3. Prevent air loss from the towing vehicle
    4. Keep the trailer spring brakes released

  14. What does a service brake priority trailer guarantee?

    1. Springs release before any air arrives
    2. Service air comes before springs release
    3. The trailer never needs its tanks drained
    4. Brakes apply only through the hand valve

  15. The trailer service brakes fail. How can you apply the trailer spring brakes?

    1. Open the trailer drain valves
    2. Push the blue tractor knob
    3. Use the trailer hand valve
    4. Close the trailer supply valve

  16. A spring-priority trailer loses its service brake air. How can you brake the trailer?

    1. Push the blue tractor knob
    2. Press the pedal harder
    3. Close the trailer supply valve
    4. Pull the trailer hand valve

  17. Which event makes trailer spring brakes apply automatically?

    1. The supply line breaks or comes off
    2. The service line is pressurized
    3. The hand valve is released
    4. The tractor reaches cut-out

  18. What protects unused couplers on the towing vehicle from dirt and water?

    1. Plastic bags over the lines
    2. Dead-end or dummy couplers
    3. The trailer hand valve
    4. The tractor protection valve

  19. Why do lines at the rear of a lead trailer have manual shut-off valves?

    1. To adjust trailer brake pressure
    2. To drain moisture from the air lines
    3. To stop air loss when unused
    4. To lock the trailer spring brakes on

  20. Can the trailer hand valve be used to hold a parked combination?

    1. No, never for parking or emergencies
    2. Yes, but only for short stops
    3. Yes, as long as the engine stays running
    4. Only when parked on level ground

  21. What is the risk with a spring brake priority trailer?

    1. It needs its own separate air compressor
    2. Its spring brakes can never be released
    3. It cannot be used in a double combination
    4. It can roll before service air is ready

  22. When do the trailer service brakes apply?

    1. Only when the tractor is parked
    2. Only when the hand valve is used
    3. Only below 60 psi
    4. Whenever the brake pedal is pressed

  23. How many air line connections join a trailer's brakes to the towing vehicle?

    1. Three: supply, service and parking
    2. One combined line
    3. Two: supply and service
    4. Four, one per trailer axle

  24. You use the hand valve and the brake pedal at the same time. What pressure reaches the trailer?

    1. Only the hand valve pressure
    2. The two added together
    3. The higher of the two
    4. The lower of the two

Trailer brakes: 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 ↗

Components

  • Trailer brake systems include air tanks and automatic spring brake application; towing vehicles need extra valves used only for the trailer.
  • A converter dolly's brake system works essentially like a trailer's.

Connecting a trailer

  • Two air connections join the trailer to the towing vehicle: the supply (emergency) line fills the trailer tanks and the service line carries service-brake pressure.
  • The connections use metal couplers called gladhands, each with a flexible seal; keep them clean and in good condition.
  • Unused couplers on the towing vehicle are protected by "dead end" or "dummy" couplers.
  • Gladhands are joined by placing the seals together at an angle and rotating them into line; once locked they take real effort to separate.
  • Couplers are usually colour-coded red for supply and blue for service, and most are shaped to fit only their match; some have no such protection.
  • Cross-connected lines make the trailer brakes work wrongly: spring brakes may not release, the towing vehicle may lose air and the service brakes will not work properly. Never drive that way.
  • Rear connectors on a trailer used for towing another trailer have manual shut-off valves to stop air loss when not in use.

Hand valve

  • Trailer service brakes apply whenever the brake pedal is pressed.
  • A trailer hand valve on the steering column or dash applies the trailer service brakes alone; if the hand valve and pedal are used together, the trailer gets the higher of the two pressures.
  • Never use the trailer hand valve for parking or emergency stops.

Trailer supply valve

  • The trailer supply valve is usually a push/pull valve with a red, eight-sided knob: push in to open and supply air to the trailer, pull out to close or when no trailer is attached.
  • Leaving it open without a trailer lets large amounts of air escape, so it closes automatically when supply-line pressure falls to between 20 and 45 psi (138–311 kPa).
  • Because air rushes out of the line, the valve usually closes well before the dash gauges fall that low.
  • If the trailer service brakes fail, you can apply the trailer spring brakes by closing the trailer supply valve.

Tractor protection valve

  • The tractor (towing vehicle) protection valve closes when the trailer supply valve closes, preventing air loss from the towing vehicle.
  • The trailer supply valve controls it; when closed, it is shut by spring force and air enters it but does not pass through.

Automatic spring brakes and priority

  • Trailer spring brakes must apply automatically when supply-line pressure drops below 60 psi (414 kPa), for example when the driver pulls the supply valve to park or when the supply line ruptures or disconnects.
  • Spring brake priority: the spring brake tank is filled first, so the trailer can roll before its service brake tank has enough air.
  • Service brake priority: the service tank fills first, ensuring service braking is available before the spring brakes release.
  • In a spring-priority trailer that loses service air, the spring brakes may stay released with no service brakes; the only way to brake the trailer is to close the trailer supply valve.
  • Ask your employer or a qualified person which priority system a trailer uses.

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