LGV Braking Systems — Questions and Answers
Question 1: What is the minimum efficiency required for the service brake on an HGV at its annual test?
- 25%
- 43%
- 50% (Correct answer)
- 60%
Correct answer: 50%
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?
- To provide additional speed
- To apply the parking brake using powerful springs when air pressure is released (Correct answer)
- To assist with steering
- To support the suspension
Correct answer: To apply the parking brake using powerful springs when air pressure is released
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?
- Nothing — the brakes work without air
- The air pressure drops and eventually the spring brakes automatically apply, stopping the vehicle (Correct answer)
- The vehicle speeds up
- Only the trailer brakes are affected
Correct answer: The air pressure drops and eventually the spring brakes automatically apply, stopping the vehicle
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?
- To show when brakes are working perfectly
- To alert the driver when air pressure in the braking system drops below a safe level (Correct answer)
- To indicate the brake pad thickness
- To show outside temperature
Correct answer: To alert the driver when air pressure in the braking system drops below a safe level
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?
- To sense the temperature of the brakes
- To automatically adjust brake pressure based on the vehicle's load, preventing wheel lock on lightly loaded axles (Correct answer)
- To count the number of brake applications
- To regulate the air compressor speed
Correct answer: To automatically adjust brake pressure based on the vehicle's load, preventing wheel lock on lightly loaded axles
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.
Question 6: How does an exhaust brake work on an HGV?
- It applies pressure to the exhaust pipe
- It restricts the exhaust flow, creating back-pressure that uses the engine as a retarder to slow the vehicle (Correct answer)
- It releases exhaust gas onto the road surface for grip
- It increases exhaust emissions to slow down
Correct answer: It restricts the exhaust flow, creating back-pressure that uses the engine as a retarder to slow the vehicle
An exhaust brake uses a butterfly valve in the exhaust manifold to restrict exhaust flow. This creates back-pressure in the engine cylinders, effectively turning the engine into an air compressor that resists rotation, providing a retarding effect without using the service brakes.
LGV Braking Systems
What is the minimum efficiency required for the service brake on an HGV at its annual test?