Understanding Horsebox Air Brakes

KPH Aeos Hybrid 4.5 in Red

Understanding horsebox air brakes

If you already use them, horsebox air brakes are something we all take for granted. It is easy to forget how daunting they can seem to customers moving up to larger vehicles and encountering them for the very first time!


Brief history

Air brakes have been around for a very long time and were first used in the railway service by George Westinghouse who patented a safer air brake on March 5, 1872. He made numerous alterations over the years to improve his air pressure brake which led to various forms of the automatic brake.

After its advantages were proven in railway use it was adopted by truck manufactures in the early 20th century.


Horsebox air brakes simplified operation

From the driver seat, horsebox air brakes have a foot brake and hand brake that basically work in the same way as your car. The only additions you can see are air tank gauges and visual/audio warnings on the dashboard. From turning on the ignition the driver will see gauges with air pressure levels of the storage tanks. If these levels are low the brakes will automatically be on and a warning symbol and buzzer will warn that the pressure is down. This is normal and the engine has an automatic air pump to fill the air brake tanks before you can use the vehicle.

For safety, it is not possible to move the vehicle until the pressure has built up and the warning symbol and buzzer have gone off.


Air handbrake/parking brake

As long as your air tank gauges are full and all warning lights and buzzers are off, it works much like you car handbrake and is simply on or off. However, if the warnings are still on and the tanks still need more air, even if you let the handbrake off, the brakes will remain on until the brake system is full of air again and the warnings go off the dashboard.


Why air brakes

The reason air bakes are so safe for larger vehicles is because in the event of a failure or fault in the brake system, rather than a runaway vehicle, any loss of air will automatically engage the brakes and safely stop the vehicle.


Load sensing valves

In addition to horsebox air brakes you will have a load sensing valve as a completely separate yet vital safety system that also deserves a little explanation. It works automatically in the background and has no gauges and no input from the driver.
If you imagine a horsebox with no horses onboard, this makes the rear of the horsebox light. Under heavy braking the rear wheels would tend to lockup and skid more easily than one with a full load. The load sensing valve monitors the weight distribution and automatically reduces the percentage of baking pressure that can be applied to the rear wheels by increasing the percentage of baking pressure that can be applied to the front wheels.

This stops the rear wheels locking up. As you add weight to the rear axle in the form of horses and tack, the load sensing valve senses this added weight increase and adds more braking pressure to the rear wheels and reduces braking pressure to the front wheels. In effect you can brake harder in an emergency and stop quicker.


MOT tips

This may be of help if you take your own vehicle for MOT. Many horseboxes fail the MOT on the rear brakes because with an unloaded horsebox, your load sensing valve will direct more braking force to the front wheels and less to the rear wheels. At test the front and rear axles are actually tested separately on a rolling road until the wheels lock up.

If they do not lock up, they fail the brake test. Therefore, it is best to place a load in the horse area (you cannot take horses!) so that the load sensing valve directs more braking force to the rear wheels and under testing they will lock up.


Anti lock brakes

Anti lock brakes have been a welcomed safety feature on all modern vehicles including horseboxes and work unseen in the background. Basically when the vehicle is moving sensors constantly check that the wheels are rotating, if one locks up the system removes braking pressure from that wheel until it rotates again.

It does this many times each second on each wheel. This clever system allows maximum breaking and shorter stopping distances. Hopefully it’s a feature you will be glad to have and never need!



To drive above 3.5 tonnes up to 7.5 tonnes you will need to have a C1 driving licence. Or if you passed your Category B (Car) driving test before 1st January 1997, you automatically have ‘grandfather rights’ that allows you to drive above 3.5 tonne up to 7.5 tonnes without having to take the C1 driving test.

See our guide for picking the right horsebox depending on your driving licence.


In conclusion

As part of your C1 driving tuition you will learn all the necessary information about air brakes. I have tried to simplify this as much as possible and it is only written to assuage some of the fears customers may have about moving up in horsebox size.


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