Braking Systems

Air brakes, ABS, retarders, brake fade and emergency stops.

What this topic covers

Air brakes, ABS, retarders, brake fade and emergency stops. The full LGV Theory Test app contains 71 questions on this topic, drawn from the official DVSA bank.

Worked examples

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

Example 1

Your lorry does not have an anti-lock braking system fitted. You may prevent the wheels from locking under heavy braking by

A. pushing the brake pedal harder until you stop
B. depressing the clutch pedal as you brake
C. rapid pumping of the brake pedal ✓ correct
D. changing down through the gears as you brake

Explanation: Use maximum pressure to the point where the wheels are about to lock, momentarily release the brake pressure, then quickly apply it again. This technique is known as cadence braking. Only use this method when the vehicle is NOT fitted with an antilock braking system. It should only be used in an emergency situation to avoid skidding.

Example 2

To help to avoid brake fade lorry drivers should ensure that

A. the air tanks are drained before journeys
B. the air pressure is correct
C. the handbrake is applied before stopping
D. the appropriate gears are engaged before downhill gradients ✓ correct

Explanation: It’s important that you engage a low gear as you approach the hill to ensure the engine is building up air and can assist with braking. If the road has a long downhill gradient this is doubly important. You should be anticipating hazards like this as you drive. Good planning and preparation will ensure that you are always in the correct gear for the situation.

Example 3

Your tractor unit has three air lines. You are connecting to a trailer with two air lines. What colour is the line you should NOT connect to the trailer?

A. Red
B. Yellow
C. Black
D. Blue ✓ correct

Explanation: When connecting three lines to a two-line trailer the third (blue) line is the one that should NOT be connected to the trailer. It is vitally important to follow the manufacturer’s advice. It may be necessary to reconnect the extra line to the tractor unit.

Example 4

You have to drive onto a muddy building site. Why should you switch on your diff-lock?

A. To make your steering lighter
B. To improve your fuel consumption
C. To increase your engine power
D. To make the wheels less likely to spin ✓ correct

Explanation: Engaging the diff-lock means that power is shared between the driven wheels. This reduces the chances of wheelspin. Remember to switch off the diff-lock as soon as you are on firm ground again, otherwise you could damage the transmission.

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