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

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

2 / 40

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

The fuel system may shut down

Spring brakes may not release

The steering may lock

The trailer ABS light will always stay on

3 / 40

What is the function of the trailer hand valve?

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

To release the trailer spring brakes only

To test the air compressor

To open the trailer supply valve

4 / 40

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

Trailer couplers or “gladhands”

Pintle hooks

Kingpins

Fifth wheel plates

5 / 40

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

To protect unused lines from dirt and water

To increase system pressure

To bypass the trailer protection valve

To reduce braking distance

6 / 40

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

Vacuum boosters and mechanical linkages

Hydraulic reservoirs and manual parking brakes

Air tanks and automatic spring brake applications

Electric actuators and drum expanders

7 / 40

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

A burned-out stop lamp

Low engine oil pressure

A flat trailer tire

A rupture or disconnection of the trailer supply line

8 / 40

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

The towing vehicle cannot build air pressure

The steering system may lose assist

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

The trailer parking brakes can never be released

9 / 40

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

The pressures are averaged

The trailer receives the lower of the two pressures

The trailer receives the higher of the two pressures

The hand valve pressure is ignored

10 / 40

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

By different mounting heights

By different thread sizes

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

By different materials

11 / 40

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

The spring brake air tank

The accessory tank

The service brake air tank

The emergency relay tank

12 / 40

Which statement about trailer spring brakes is correct?

They operate only when the trailer is empty

They cannot be applied automatically

They provide both parking and emergency braking on trailers

They are used only for normal service braking

13 / 40

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

To automatically balance brake pressures

To isolate the tractor from the trailer at all times

To increase service brake application speed

To prevent air loss when lines are not in use

14 / 40

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

To operate only the parking brakes

To carry air to fill the trailer’s tanks

To ventilate the trailer air system

To signal the ABS system

15 / 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 apply the towing vehicle’s parking brakes

To manually adjust the brake balance

16 / 40

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

To power auxiliary trailer equipment

To carry pressure for the trailer service brakes

To vent air from the brake chambers

To supply air to the air suspension only

17 / 40

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

To avoid overcharging the batteries

To keep the lights from staying on

To prevent fuel from being wasted

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

18 / 40

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

Leave it in any position

Push it in to keep air flowing

Pull it out to close the valve

Turn it halfway to “neutral”

19 / 40

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

To improve fuel economy

To maintain a leak‑free connection

To reduce engine noise

To prevent electrical interference

20 / 40

When are the trailer service brakes applied during normal operation?

Only when the parking brakes are set

Whenever the brake pedal is pressed

Only when the trailer hand valve is used

Only during emergency stops

21 / 40

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

A flexible seal

A gasket-less design

A locking cotter pin

A rigid metal insert

22 / 40

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

Inversion valve

Relay valve

Foot valve

Tractor protection valve

23 / 40

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

Between 415 and 550 kPa (60–80 psi)

Between 690 and 830 kPa (100–120 psi)

Below 70 kPa (10 psi)

Between 138 and 311 kPa (20–45 psi)

24 / 40

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

The air dryer shuts off

The parking brakes release

The spring brakes begin to apply

The trailer lights dim

25 / 40

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

Main line and return line

Parking line and emergency line

Supply (emergency) line and service line

Primary line and secondary line

26 / 40

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

It closes to trap air in the tractor tanks

It opens to allow air to the trailer service line

It vents all air to the atmosphere

It bypasses the service line

27 / 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 turns on the headlights

The driver shifts into a higher gear

The trailer lights fail

28 / 40

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

It eliminates the need for gladhands

It allows operation with no air in the tanks

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

It automatically inflates trailer tires

29 / 40

When connecting gladhands, how are they joined?

Clamp them with a separate tool

Push straight together without rotation

Screw them together with a threaded collar

Place the sealing surfaces at an angle and rotate into alignment

30 / 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

Spring brakes may remain released while no service brakes are available

The service brakes may overheat

31 / 40

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

By pressing the clutch pedal

By switching off the ignition

By using the accelerator pedal

By using (pulling) the trailer supply valve

32 / 40

When do trailer spring brakes have to apply automatically?

When the fuel tank is low

When the headlights are turned off

Whenever vehicle speed exceeds a set limit

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

33 / 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 steering wheel locks

The fuel system shuts down

34 / 40

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

Smoothing brake applications

Brake testing

Parking or emergency stops

Normal service braking

35 / 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

Ignore the difference because it is unimportant

Consult their employer or a qualified person

Assume all trailers use service brake priority

36 / 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 closing (pulling out) the trailer supply valve

By shifting to neutral

By turning on the headlights

37 / 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

38 / 40

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

The trailer supply valve

The hand valve

The parking brake lever

The brake pedal

39 / 40

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

Red, eight‑sided knob

Blue, square knob

Green, triangular knob

Yellow, round knob

40 / 40

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

The trailer brakes will not work correctly

Only the ABS will be affected

Only the parking brake will function

The trailer brakes will work normally

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.