LGV Braking Systems

Question 1

What is the minimum efficiency required for the service brake on an HGV at its annual test?

Accepted Answer

50%

Answer

The minimum service brake efficiency required at the annual test is 50% for vehicles first used on or after 1 January 2012. Older vehicles may have a requirement of 43%. The parking brake must achieve at least 16% efficiency.

Question 2

What is the function of the spring brake on an HGV?

Accepted Answer

To apply the parking brake using powerful springs when air pressure is released

Answer

Spring brakes use powerful compression springs to apply the parking brake. When air pressure is released (by the parking brake valve), the springs push against the brake diaphragm to apply the brakes. They also act as an emergency brake if air pressure is lost.

Question 3

What happens to the braking system if the air compressor fails on an HGV?

Accepted Answer

The air pressure drops and eventually the spring brakes automatically apply, stopping the vehicle

Answer

If the air compressor fails, stored air in the tanks gradually depletes through brake applications. Warning devices activate at approximately 4.5 bar. If pressure drops below the spring brake release threshold (around 3.5 bar), the spring brakes automatically apply.

Question 4

What is the purpose of the brake pressure warning indicator?

Accepted Answer

To alert the driver when air pressure in the braking system drops below a safe level

Answer

The brake pressure warning indicator (typically a red warning light and/or buzzer) alerts the driver when air pressure in the braking system drops below a safe level (approximately 4.5 bar). If this activates, the driver must stop safely and not continue until the issue is resolved.

Question 5

What is the function of the load sensing valve (LSV) in the braking system?

Accepted Answer

To automatically adjust brake pressure based on the vehicle's load, preventing wheel lock on lightly loaded axles

Answer

The load sensing valve automatically adjusts brake pressure to each axle based on the actual load being carried. A fully loaded vehicle needs more brake pressure than an empty one. Without an LSV, empty or lightly loaded vehicles could lock their wheels during braking.

DVSA UK Driving Theory Practice Test

DVSA UK Driving Theory Practice Test

LGV Braking Systems