Service Brake

The foot control operates the service brake. It controls the vehicle’s speed and brings it to a safe stop.  

If the brake pedal is hard when pressed, it could indicate a fault with the vacuum pump or a leaking connection allowing air into the vacuum system.

If this occurs, it is important to get the fault checked before proceeding.

The minimum braking efficiency for the service brake should be no less than 50%

Secondary Brake

A secondary brake works on fewer wheels, which means it has a reduced level of effectiveness. If your service brake fails, you can use your secondary brake in conjunction with the parking brake to slow and eventually stop the vehicle

The minimum braking efficiency for the secondary brake should be no less than 25%

Parking Brake

The parking brake or handbrake must always be applied when you leave the vehicle.

The minimum braking efficiency for the parking brake should be no less than 16%

Engaging the diff-lock means that the driven wheels are locked together. This reduces the likelihood of wheel spin.

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.

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.

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. Don’t start or continue your journey until the fault has been repaired.

If the brake pedal is hard to press, this could mean a loss of vacuum or a fault in the vacuum pump. You shouldn’t drive the vehicle until the fault has been repaired.

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

In frosty weather, to prevent moisture from freezing in the air-brake system, you should make sure that you drain the air tanks daily. Most modern vehicles have an automatic draining system, which should be checked regularly.

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.


An endurance brake or retarder, as it is commonly known, is a system for controlling the vehicle’s speed without using the footbrake. It replaces some of the functions of friction-based brakes, which are susceptible to brake fade on long downhill slopes.

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.

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.

Anti-locking braking system

Just as the wheels are about to lock, the sensor control releases the brakes and immediately applies them repeatedly in quick succession, enabling the driver to steer whilst braking.

If you’re driving a vehicle with anti-lock brakes and you feel the vehicle beginning to skid, you should keep your foot firmly on the brake pedal until the vehicle stops.


Use the vehicles’ gears to help in the braking process when descending hills, especially if the vehicle is loaded.

You should use a low gear or endurance brake (if you have one), which will increase the effectiveness of the engine braking

The revs should ideally be kept in the blue band on your rev counter for maximum braking efficiency.


Continual use of the brakes could cause them to fade, which makes them less effective. Make sure that you’re in the correct gear before you negotiate downhill stretches of road.

low gear will assist with braking and help prevent the lorry from gaining momentum as you negotiate a descent.


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 them effectively.


Escape lanes are found on steep downhill sections of road. They’re designed to give a ‘run-off’ area, usually straight ahead, to allow vehicles to stop in an emergency. This is usually when brakes have faded or failed on large vehicles


A two-line system consists of the:

Emergency line and service line –  It’s important to understand how to safely connecting brake systems and mixing two and three line systems

Three line system

Emergency line = red
Service line = yellow
Auxiliary line = blue

Two line system

Emergency line = red
Service line = yellow

When connecting three lines to a two-line trailer, the blue line is the one that should NOT be connected to the trailer.


A combination of sharp braking and excessive steering can cause your vehicle to become unstable. Jack-knifing is more likely to occur when the vehicle is empty, unladen or not travelling in a straight line. 

Severe braking or selection of a gear too low for your road speed can cause the tractor unit to be pushed by the semi-trailer pivoting around the coupling (fifth wheel).


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 to preventing these forces from acting together.