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LGV study notes

Braking Systems

Air brakes, ABS, endurance brakes, brake fade, maintenance and freezing conditions.

What this topic covers

Air brakes, ABS, endurance brakes, brake fade, maintenance and freezing conditions. The full LGV & HGV Theory Test & Toolkit app contains 67 questions on this topic.

In the app's syllabus: Types of brake, Maintenance and inspection, Connection & proper use of the brakes, Tailgating, Freezing conditions, Anti-lock brakes.

Worked examples

Here are a few practice questions from this topic. Try answering each before reading the explanation.

Example 1

When is trailer swing most likely to occur on a lorry and drawbar combination?

A. While you're braking on a bend ✓ correct
B. While you're using an endurance brake
C. While you're steering at slow speed and fully loaded
D. While you're moving off at an angle

Explanation: The forces that act on your vehicle if you brake while cornering can lead to instability and may cause trailer swing. You should try to plan ahead so that you brake smoothly and in good time as you approach a bend.

Example 2

You're driving a tractor unit fitted with 2 air lines. You want to couple to a trailer that has 3 air-line couplings. How should the air lines be connected?

A. The trailer auxiliary line should be left unconnected ✓ correct
B. The trailer service line should be left unconnected
C. Only the service line should be connected
D. Only the auxiliary line should be connected

Explanation: A two-line system consists of the emergency line (red) and the service line (yellow). You need to understand the rules that apply to safe connection of brake systems and mixing two-line and three-line systems.

Example 3

What could prevent air pressure from building up in an air-brake system in frosty weather?

A. Moisture in the air may form bubbles in the brake fluid
B. The air will contract, reducing the pressure
C. The dampness may cause valves to rust
D. Moisture drawn in with the air may freeze and cause a blockage ✓ correct

Explanation: When air is compressed, moisture condenses and collects in the air tanks. This can find its way along the network of pipes connected to the brakes. In frosty weather, the moisture can freeze in the pipes, blocking them completely. On modern vehicles, the air is dried before it's compressed and the air tanks drain automatically to shed any moisture. On older vehicles, the air tanks need draining manually.

Example 4

You're driving a vehicle fitted with anti-lock brakes. What should you do when braking in an emergency?

A. ‘Pump’ the brake pedal harshly
B. Apply minimum force to the brake pedal
C. Use the exhaust brake (retarder) before the footbrake
D. Apply firm, continuous pressure to the brake pedal ✓ correct

Explanation: Applying a firm, continuous brake pressure will allow the anti-lock braking system (ABS) to function properly. However, ABS is becoming increasingly sophisticated, so you should read the vehicle manufacturer's handbook for its advice about using the system.

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More LGV study notes

Get all 67 questions on this topic in the LGV & HGV Theory Test & Toolkit app.