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

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

To ventilate the trailer air system

To signal the ABS system

To operate only the parking brakes

To carry air to fill the trailer’s tanks

2 / 40

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

A burned-out stop lamp

A rupture or disconnection of the trailer supply line

Low engine oil pressure

A flat trailer tire

3 / 40

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

To avoid overcharging the batteries

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

To keep the lights from staying on

To prevent fuel from being wasted

4 / 40

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

It vents all air to the atmosphere

It opens to allow air to the trailer service line

It bypasses the service line

It closes to trap air in the tractor tanks

5 / 40

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

To improve fuel economy

To prevent electrical interference

To reduce engine noise

To maintain a leak‑free connection

6 / 40

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

Push it in to keep air flowing

Turn it halfway to “neutral”

Leave it in any position

Pull it out to close the valve

7 / 40

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

To carry pressure for the trailer service brakes

To vent air from the brake chambers

To power auxiliary trailer equipment

To supply air to the air suspension only

8 / 40

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

The trailer parking brakes can never be released

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

The towing vehicle cannot build air pressure

The steering system may lose assist

9 / 40

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

The parking brakes release

The spring brakes begin to apply

The air dryer shuts off

The trailer lights dim

10 / 40

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

Foot valve

Tractor protection valve

Inversion valve

Relay valve

11 / 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 hand valve pressure is ignored

The trailer receives the higher of the two pressures

The pressures are averaged

12 / 40

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

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

By different thread sizes

By different mounting heights

By different materials

13 / 40

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

Consult their employer or a qualified person

Test it by disconnecting lines while moving

Assume all trailers use service brake priority

Ignore the difference because it is unimportant

14 / 40

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

They support the trailer weight

They help prevent cross‑connection through design and colour coding

They lock the kingpin in place

They equalize tire pressures

15 / 40

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

By switching off the ignition

By pressing the clutch pedal

By using the accelerator pedal

By using (pulling) the trailer supply valve

16 / 40

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

Fifth wheel plates

Trailer couplers or “gladhands”

Pintle hooks

Kingpins

17 / 40

When do trailer spring brakes have to apply automatically?

When the fuel tank is low

When the headlights are turned off

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

Whenever vehicle speed exceeds a set limit

18 / 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 415 and 550 kPa (60–80 psi)

Between 138 and 311 kPa (20–45 psi)

19 / 40

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

Supply (emergency) line and service line

Main line and return line

Primary line and secondary line

Parking line and emergency line

20 / 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 shifting to neutral

By turning on the headlights

By closing (pulling out) the trailer supply valve

By pressing the accelerator

21 / 40

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

The spring brake air tank

The suspension air tank

The accessory air tank

The service brake air tank

22 / 40

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

To isolate the tractor from the trailer at all times

To prevent air loss when lines are not in use

To increase service brake application speed

To automatically balance brake pressures

23 / 40

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

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

It automatically inflates trailer tires

24 / 40

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

The trailer lights fail

The driver parks and pulls out the trailer supply valve

The driver shifts into a higher gear

The driver turns on the headlights

25 / 40

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

The steering wheel locks

The fuel system shuts down

The trailer supply valve closes automatically

The trailer hand valve applies

26 / 40

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

The trailer brakes will work normally

Only the parking brake will function

Only the ABS will be affected

The trailer brakes will not work correctly

27 / 40

When connecting gladhands, how are they joined?

Clamp them with a separate tool

Push straight together without rotation

Place the sealing surfaces at an angle and rotate into alignment

Screw them together with a threaded collar

28 / 40

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

Spring brakes may not release

The steering may lock

The trailer ABS light will always stay on

The fuel system may shut down

29 / 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

The ABS may cycle continuously

The service brakes may overheat

Spring brakes may remain released while no service brakes are available

30 / 40

What is the function of the trailer hand valve?

To release the trailer spring brakes only

To test the air compressor

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

To open the trailer supply valve

31 / 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

32 / 40

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

To increase system pressure

To reduce braking distance

To protect unused lines from dirt and water

To bypass the trailer protection valve

33 / 40

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

The accessory tank

The service brake air tank

The emergency relay tank

The spring brake air tank

34 / 40

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

A locking cotter pin

A flexible seal

A gasket-less design

A rigid metal insert

35 / 40

Which statement about trailer spring brakes is correct?

They operate only when the trailer is empty

They provide both parking and emergency braking on trailers

They are used only for normal service braking

They cannot be applied automatically

36 / 40

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

Yellow, round knob

Red, eight‑sided knob

Blue, square knob

Green, triangular knob

37 / 40

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

Vacuum boosters and mechanical linkages

Electric actuators and drum expanders

Air tanks and automatic spring brake applications

Hydraulic reservoirs and manual parking brakes

38 / 40

When are the trailer service brakes applied during normal operation?

Only during emergency stops

Only when the parking brakes are set

Whenever the brake pedal is pressed

Only when the trailer hand valve is used

39 / 40

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

Brake testing

Parking or emergency stops

Normal service braking

Smoothing brake applications

40 / 40

What is the main function of the trailer supply valve?

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

To drain the air tanks

To manually adjust the brake balance

To apply the towing vehicle’s parking brakes

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.