Chapter 5: Trailer Brake Subsystem

The “Trailer Brake Subsystem” chapter in Ontario’s air brake test guide focuses on the key components and operation of trailer brakes in commercial vehicles.

Ontario Air Brake Test - Chapter 5: Trailer Brake Subsystem

1 / 40

How are trailer supply and service couplers usually identified to prevent cross‑connection?

By different mounting heights

By colour coding (red for supply, blue for service)

By different materials

By different thread sizes

2 / 40

Which of the following problems can result from cross‑connected trailer lines?

Spring brakes may not release

The trailer ABS light will always stay on

The steering may lock

The fuel system may shut down

3 / 40

What is the main function of the trailer supply valve?

To manually adjust the brake balance

To apply the towing vehicle’s parking brakes

To supply air to the trailer’s brake system through the supply line

To drain the air tanks

4 / 40

Why is it important to close the trailer supply valve when no trailer is attached?

To keep the lights from staying on

To prevent a large amount of air from escaping from the towing vehicle’s system

To prevent fuel from being wasted

To avoid overcharging the batteries

5 / 40

What is the primary function of the service line to the trailer?

To carry pressure for the trailer service brakes

To supply air to the air suspension only

To power auxiliary trailer equipment

To vent air from the brake chambers

6 / 40

What is the primary function of the supply (emergency) line to the trailer?

To operate only the parking brakes

To ventilate the trailer air system

To carry air to fill the trailer’s tanks

To signal the ABS system

7 / 40

What else can cause the trailer spring brakes to apply automatically?

A rupture or disconnection of the trailer supply line

A flat trailer tire

Low engine oil pressure

A burned-out stop lamp

8 / 40

What is a safety disadvantage of a spring brake priority system?

The trailer parking brakes can never be released

The towing vehicle cannot build air pressure

The trailer can be towed before the service brakes have enough pressure to operate

The steering system may lose assist

9 / 40

Which statement about trailer spring brakes is correct?

They cannot be applied automatically

They are used only for normal service braking

They operate only when the trailer is empty

They provide both parking and emergency braking on trailers

10 / 40

Which of the following can cause trailer supply line pressure to drop and apply the spring brakes?

The driver parks and pulls out the trailer supply valve

The driver shifts into a higher gear

The trailer lights fail

The driver turns on the headlights

11 / 40

Why must gladhands and their seals be kept clean and in good condition?

To reduce engine noise

To maintain a leak‑free connection

To prevent electrical interference

To improve fuel economy

12 / 40

What must the driver do with the trailer supply valve when no trailer is attached?

Turn it halfway to “neutral”

Push it in to keep air flowing

Leave it in any position

Pull it out to close the valve

13 / 40

In an emergency when the trailer service brakes fail, how can the trailer spring brakes be applied?

By using (pulling) the trailer supply valve

By pressing the clutch pedal

By using the accelerator pedal

By switching off the ignition

14 / 40

When connecting gladhands, how are they joined?

Place the sealing surfaces at an angle and rotate into alignment

Screw them together with a threaded collar

Push straight together without rotation

Clamp them with a separate tool

15 / 40

What important safety feature does a service brake priority system provide?

It automatically inflates trailer tires

It ensures service brakes have enough air before spring brakes can be released

It eliminates the need for gladhands

It allows operation with no air in the tanks

16 / 40

What happens to the tractor protection valve when the trailer supply valve is opened?

It bypasses the service line

It closes to trap air in the tractor tanks

It vents all air to the atmosphere

It opens to allow air to the trailer service line

17 / 40

What feature on each gladhand helps provide a leak‑free connection?

A locking cotter pin

A gasket-less design

A rigid metal insert

A flexible seal

18 / 40

What component on the towing vehicle prevents air loss when there is no trailer attached?

Tractor protection valve

Foot valve

Relay valve

Inversion valve

19 / 40

At approximately what pressure in the trailer supply line does the trailer supply valve close automatically?

Between 690 and 830 kPa (100–120 psi)

Below 70 kPa (10 psi)

Between 138 and 311 kPa (20–45 psi)

Between 415 and 550 kPa (60–80 psi)

20 / 40

When do trailer spring brakes have to apply automatically?

When the headlights are turned off

Whenever vehicle speed exceeds a set limit

When the fuel tank is low

When air pressure in the trailer supply line drops below 414 kPa (60 psi)

21 / 40

What is the purpose of manual shut‑off valves at the rear of a trailer with extra connectors?

To increase service brake application speed

To automatically balance brake pressures

To prevent air loss when lines are not in use

To isolate the tractor from the trailer at all times

22 / 40

In a spring brake priority system with loss of air to the service brakes, what risk may occur?

The parking brakes may drag slightly

Spring brakes may remain released while no service brakes are available

The ABS may cycle continuously

The service brakes may overheat

23 / 40

If a spring brake priority trailer loses service brake air and the spring brakes stay released, how can the driver apply brakes on the trailer?

By pressing the accelerator

By shifting to neutral

By turning on the headlights

By closing (pulling out) the trailer supply valve

24 / 40

What is the normal colour and shape of the trailer supply valve control in the cab?

Blue, square knob

Red, eight‑sided knob

Green, triangular knob

Yellow, round knob

25 / 40

For what purpose must the trailer hand valve never be used?

Parking or emergency stops

Normal service braking

Brake testing

Smoothing brake applications

26 / 40

What should a driver do to identify whether a trailer uses spring brake or service brake priority?

Test it by disconnecting lines while moving

Consult their employer or a qualified person

Assume all trailers use service brake priority

Ignore the difference because it is unimportant

27 / 40

What happens automatically if air escapes from an open trailer supply line or a trailer breaks loose?

The trailer hand valve applies

The trailer supply valve closes automatically

The fuel system shuts down

The steering wheel locks

28 / 40

In a “service brake priority” trailer system, which air tank is filled first?

The service brake air tank

The suspension air tank

The spring brake air tank

The accessory air tank

29 / 40

What components are included in a trailer’s air brake subsystem?

Air tanks and automatic spring brake applications

Vacuum boosters and mechanical linkages

Electric actuators and drum expanders

Hydraulic reservoirs and manual parking brakes

30 / 40

How is brake pressure to the trailer determined if both the brake pedal and hand valve are used together?

The trailer receives the lower of the two pressures

The pressures are averaged

The trailer receives the higher of the two pressures

The hand valve pressure is ignored

31 / 40

What is the function of the trailer hand valve?

To test the air compressor

To release the trailer spring brakes only

To open the trailer supply valve

To apply the trailer service brakes independent of the towing vehicle’s brakes

32 / 40

What controls the opening and closing of the tractor protection valve?

The hand valve

The trailer supply valve

The brake pedal

The parking brake lever

33 / 40

When are the trailer service brakes applied during normal operation?

Only when the trailer hand valve is used

Only during emergency stops

Only when the parking brakes are set

Whenever the brake pedal is pressed

34 / 40

In a “spring brake priority” trailer system, which air tank is filled first?

The accessory tank

The service brake air tank

The spring brake air tank

The emergency relay tank

35 / 40

What is the purpose of “dead end” or “dummy” couplers on the towing vehicle?

To bypass the trailer protection valve

To increase system pressure

To protect unused lines from dirt and water

To reduce braking distance

36 / 40

What are the two air line connections between a towing vehicle and a trailer called?

Supply (emergency) line and service line

Parking line and emergency line

Main line and return line

Primary line and secondary line

37 / 40

What component is used to connect the trailer’s air lines to the towing vehicle?

Kingpins

Pintle hooks

Trailer couplers or “gladhands”

Fifth wheel plates

38 / 40

What happens if the trailer air lines are cross‑connected?

The trailer brakes will not work correctly

The trailer brakes will work normally

Only the parking brake will function

Only the ABS will be affected

39 / 40

In some spring brake priority systems, what happens if there is air loss to the service brakes?

The trailer lights dim

The parking brakes release

The air dryer shuts off

The spring brakes begin to apply

40 / 40

What is a key function of trailer couplers (gladhands) besides connecting air lines?

They help prevent cross‑connection through design and colour coding

They equalize tire pressures

They support the trailer weight

They lock the kingpin in place

Your score is

This section explains how air is supplied from the tractor to the trailer, the function of the service and emergency brake lines, and the importance of the tractor protection valve. Drivers must understand how to check for proper brake application, air loss, and emergency response. Proper knowledge ensures safe coupling and uncoupling, preventing brake failure and accidents. Mastery of this chapter is essential for passing the Ontario air brake test and ensuring safe operation of vehicles with trailers.

Test yourself with the Ontario air brake practice test questions until you can clearly explain the concepts without referring to the notes.