LGV/HGV Theory Test Practice

When your steering wheels are locked - as can happen in a skid - you cannot steer the vehicle. Anti-lock brakes help you to keep steering control while you're braking heavily, but they’re not a substitute for good forward planning. Anticipation and planning ahead will minimise the risk of skidding more effectively than relying on your braking system.

Even though the warning light is showing, there may be sufficient pressure to release the parking brake. However, you must not do this, because there may not be enough air pressure to operate the service brake.

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.

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.

If a warning buzzer or light alerts you to a loss of air pressure, you should stop on the side of the road immediately. You should have enough air in reserve to allow you to do this safely. Do not start or continue your journey until the fault has been repaired.

Continually using the brakes could cause them to overheat. When that happens, they become less effective. This is called brake fade, and it occurs most often when the driver has to control the vehicle's speed on a long downhill gradient. To help prevent brake fade, use the endurance brake (retarder) and a lower gear to keep the vehicle's speed in check, rather than the wheel-mounted brakes.

The endurance brake (retarder) usually operates by applying resistance, via the transmission, to the vehicle's driven wheels. If the road is slippery, the tyres may lose grip and the wheels will lock.

The differential allows the drive wheels to rotate at different speeds; this is necessary for the vehicle to negotiate corners and bends. On a slippery surface, however, it can cause problems: if one wheel has less traction, it can spin, resulting in a complete loss of drive. The 'diff-lock' prevents the wheels from turning independently and so helps to stop this happening.

Continuous use of the brakes on a long downhill gradient can cause them to overheat and become ineffective. This is called brake fade. You should use a low gear to allow the engine's braking effect to help control your speed. Also use the vehicle's retarder, if fitted. As well as reducing the likelihood of brake fade, this helps to keep air pressure in the tanks.

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.

The endurance brake (retarder) slows the vehicle without the use of the wheel-mounted brakes. However, care should be taken in slippery conditions, when the retarder's effect can be powerful enough to cause a skid.

There are usually 4 coloured bands on the rev counter: green, amber, blue and red. The blue band will allow you to make the best use of engine braking. Try to keep in this band when going down a steep hill.

A loose brake pedal would suggest a serious loss of fluid from the hydraulic system. Do not drive your vehicle until the fault has been checked and fixed by a qualified person.

Heavy braking can cause an articulated vehicle to become unstable and may even lead to jack-knifing; this is more likely to occur when the trailer is unladen.

Air-brake systems are fitted with a warning device that operates if the air pressure in the tanks drops below a safe level. There may be a warning buzzer and/or pressure gauges. You must be aware of the function of all gauges on your vehicle, and check them as you drive.

Plan well ahead to allow you to brake normally. Do not rely on anti-lock brakes to make up for poor driving.

By using a retarder, the life of the brake linings is extended. This is because a retarder works by slowing the vehicle without using the wheel-mounted brakes.

It's dangerous to drive a vehicle with low air pressure. To alert you to this, all vehicles are fitted with a warning light and/or buzzer. Do not move your vehicle if this warning shows or sounds.

The way you drive matters. The safety and comfort of your passengers is your priority; if you have the correct attitude when you're driving, your passengers will be assured of a comfortable and pleasant journey.
Good forward planning and anticipation will help you avoid having to turn or brake suddenly.

Select an appropriate gear in good time and, if your vehicle has a dashboard-mounted lever, apply the endurance brake (retarder) in stages. Braking too much, too soon may result in locking your drive wheels.

When your vehicle has anti-lock brakes and you need to stop in an emergency, you should keep your foot firmly on the brake pedal until the vehicle stops; this will allow the system to work. Although anti-lock brakes contribute to safety, they do not reduce your stopping distance or take away the need to drive with good planning and anticipation.

Continuous use of the brakes will cause them to overheat, and in extreme cases they'll become ineffective. When you're going downhill, the momentum of your vehicle will cause you to gather speed very quickly. Use the endurance brake (retarder) and stay in a low gear to reduce the need to use the brakes.

The service brake is usually operated by the brake pedal. It's used to control the speed of the vehicle and to bring it to a halt safely. It may also incorporate an anti-lock braking system.

Air-brake systems rely on an engine-driven compressor to keep the reservoir tanks at their operating pressure. Coasting downhill and relying on the brakes to control your speed could result in the loss of sufficient air pressure to operate the brakes effectively.

Anti-lock brakes are a driver aid, and can help you maintain steering control while braking. However, you must not rely on them to get you out of trouble or expect to be able to make sudden direction changes if you're going too fast.

When a warning light or device indicating a loss of brake pressure comes on, you must stop safely and get the fault put right immediately. The safety of you, your load, your passengers and all other road users is at risk.

Anti-lock braking systems sense when the wheels are about to lock. The sensor control releases the brakes and immediately applies them again. It will do this many times per second. This both prevents skidding and allows the driver to steer - something that's impossible when the steering wheels are skidding.

Every vehicle fitted with anti-lock brakes must have a warning light in the cab of the vehicle. The warnings may differ between manufacturers, but on all types the warning light should come on when the ignition is switched on, and turn off when the vehicle reaches a speed of 6 mph (10 km/h). If the light stays on, there may be a fault; you should have the brakes checked before continuing your journey.

You should make sure that you drain the air tanks daily to avoid moisture in the system. Most modern vehicles have an automatic draining system, which should be checked regularly.

Moisture in the braking system can cause serious problems, especially in cold weather, when it could freeze and stop the brakes from working. Most modern vehicles have a system that drains automatically, but make sure you know which system is fitted to any vehicle that you drive.

Even though a warning device indicates low air-brake pressure, you may still be able to release the parking brake. If you do this and start moving, the service brake may be ineffective and you may not be able to stop.

Large vehicles can be fitted with various types of endurance brake (retarder). Make sure you're familiar with how the system operates on the vehicle you're using. Do not risk a crash or collision through ignorance.

Brake fade occurs when the brakes overheat. Good forward planning and correct use of the gears when descending hills, together with proper use of the endurance brake (retarder), can help to prevent brake fade.

Modern anti-lock braking systems rely on electronics for their operation. A warning light on the dashboard shows that the system is operating. This light will usually go out when the vehicle reaches about 6 mph (10 km/h).

You must always disengage the diff-lock as soon as the vehicle is moving. The differential allows the rear wheels to revolve at different speeds, and this allows the vehicle to follow a curved path. Attempting to turn with the diff-lock engaged could result in the vehicle continuing straight on.

Plan ahead and use your endurance brake (retarder) to help keep your speed in check on long downhill gradients. This can help to prevent your brakes from overheating and fading.

Air-brake systems use air from the atmosphere, which contains moisture. The moisture condenses under pressure, resulting in water in the system. Make sure the air tanks are drained daily to remove any water and to help prevent the system from freezing in cold weather.

Continuous use of the brakes can result in them overheating and losing their effectiveness. This is known as 'brake fade' and it's most likely on long downhill gradients.

An exhaust brake alters the engine's exhaust flow, using it to help control the vehicle's speed. It's most efficient when the engine is running at a high speed and the vehicle is in a low gear, such as when descending a long hill. Using the exhaust brake can relieve the service brakes, preventing them from overheating and fading.

If the brake pedal is hard to press, this could mean a loss of air pressure or a fault in the air compressor. You should not drive the vehicle until the fault has been repaired.

If you're driving an articulated vehicle or a trailer combination, it's vital that you understand the rules that apply to coupling and uncoupling the brake lines. If you take a practical driving test with a trailer, you'll be expected to demonstrate this during the test. The lines are colour-coded: red is the emergency line, blue is the auxiliary line and yellow is the service line. They must be connected strictly following the correct procedure.

An endurance brake can help to control your speed on long downhill slopes without using the service brake. Reducing the use of the service brake helps to prevent the brakes from overheating and reduces the risk of brake fade.

The red emergency line is common to both two-line and three-line brake systems. The other colours are
• blue - auxiliary
• yellow - service.

The blue (auxiliary) line is not used when connecting to a two-line system. Follow the manufacturer's instructions about what to do with this third line.

If you have to drive through a flood, do so with caution. Once out of the flood, you need to test your brakes to make sure they're working properly. To do this, check what's behind you, then drive in a low gear with the brakes gently applied.

When a vehicle changes direction, forces are applied to the vehicle and its load. Braking at the same time can result in additional forces being introduced, and this can lead to a loss of control. For this reason, braking should be carried out while driving in a straight line, so preventing these forces from acting together.

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.

When approaching a long hill, you should take note of any early warning signs. Reduce your speed and select the appropriate gear in good time. Using an endurance brake (retarder), if fitted, will help control your speed.

The most powerful and effective brake on the vehicle is the service brake, and this should be used in normal circumstances. Well-maintained brakes should apply an even pressure to all the wheels, providing an efficient, controlled stop.

Severe braking or selecting a gear too low for your road speed can cause the tractor unit to slow more quickly than the trailer. The trailer can then pivot around the coupling (fifth wheel), causing the vehicle to jack-knife. Jack-knifing is more likely to occur with an unladen vehicle, particularly on a bend or curve.

The air compressor provides high-pressure air that assists operation of the hydraulic brakes. This reduces the amount of pressure you need to apply to the brake pedal. Without this assistance, it's extremely difficult to press the brake pedal down.

It's important to check all fluid levels during your daily checks, but it's especially important to remember the hydraulic fluid reservoir. A loss of fluid could lead to brake failure.

Always be prepared to give way to overtaking lorries or buses. Maintaining your speed will only block the motorway to other traffic unnecessarily. You may find yourself in a similar situation if the limiter on your vehicle does not allow you enough speed to successfully overtake another vehicle.

Engaging the diff-lock means that the driven wheels are locked together. This reduces the likelihood of wheel spin. Remember to switch off the diff-lock as soon as you're on firm ground again, otherwise you could damage the transmission.

Engaging a lower gear and using engine braking will help you to control your speed as you're going downhill. This reduces the demands on the brakes, and helps to prevent them from overheating. This will reduce the likelihood of brake fade.

Before driving a vehicle fitted with anti-lock brakes, refer to the handbook. This will give you details of the manufacturer's recommended method of use. Remember, anti-lock brakes are a driver aid, but they cannot replace your driving skills.

The red emergency line is common to both two-line and three-line braking systems. Always set the parking brake before disconnecting any brake line.

As with stopping in any other situation, if you have to make a stop on an uphill gradient, wait until the vehicle has come to a halt before applying the parking brake.

Using the brakes to control a lorry's speed as it's driven down a hill will cause them to get hot. If they overheat, they can become less effective - a condition known as brake fade. By selecting a low gear, the engine's resistance will slow the lorry. This reduces the demands on the brakes, making them less likely to overheat.

Endurance brakes or 'retarders' help to control a vehicle's speed. They operate by applying resistance to slow the vehicle without using the wheel brakes, so avoiding brake fade. The methods used include: increased engine braking, exhaust braking and transmission-mounted devices.

It's important to apply the footbrake before you engage 'D' (or drive), so your vehicle does not creep forward or roll back out of control when you're about to move away. This can be dangerous; for example, if there's another road user close behind.

A fault in the air compressor could be the cause of a stiff brake pedal. There could also be a leaking connection allowing air to leak out of the compressed air system. Do not continue your journey until the fault has been fixed.

When connecting a three-line tractor unit to a two-line trailer, the blue line is the one that should not be connected to the trailer. This is the auxiliary line. Always follow the manufacturer's advice: it may be necessary to reconnect the extra line to the tractor unit.