Compressed air plays a vital role in the operation of a modern heavy goods vehicle. While braking is its most important application, compressed air can also be used for suspension, trailer systems and other auxiliary functions.
For drivers, technicians and fleet operators, understanding the basic layout of an HGV air system can make it easier to recognise developing faults and identify which components may need further investigation.
Although modern systems can contain numerous valves, sensors and electronically controlled components, the core principle is straightforward. The vehicle takes atmospheric air, compresses it, removes moisture and contaminants, stores it under pressure and distributes it to the systems that need it.
Why do HGVs use compressed air?
Passenger cars generally use hydraulic braking systems, where brake fluid transmits force from the brake pedal to the brakes.
Heavy commercial vehicles operate differently.
Because of their size, weight and braking requirements, HGVs commonly use compressed air as the primary means of operating their braking systems.
Compressed air can be stored in reservoirs and distributed across multiple braking circuits, making it well suited to large vehicles and tractor-trailer combinations.
One important feature is that parking brakes on heavy vehicles commonly use powerful mechanical springs. Air pressure holds those springs off while the vehicle is moving. If pressure falls too low, the system is designed to move towards a braking state rather than simply losing braking ability.
The basic journey of air through an HGV
Although layouts vary between manufacturers, compressed air generally follows a route similar to this:
Atmospheric air → compressor → air dryer → protection valves → reservoirs → control valves → brake or auxiliary system → actuator
Each stage has a specific role.
Problems anywhere in this chain can result in symptoms such as slow air build-up, low-pressure warnings, leaks, braking problems or suspension faults.
The air compressor
The compressor is the starting point of the system.
Usually driven by the engine, it draws in atmospheric air and compresses it before sending it through the air-treatment system.
As the vehicle uses compressed air, reservoir pressure falls. The compressor then replenishes the supply until the correct operating pressure is restored.
If air takes unusually long to build, the compressor may be one possible cause, but it should not be assumed to be at fault immediately. Leaks, faulty valves, air dryer problems and trailer faults can all produce similar symptoms.
We explore this further in our guide to signs your HGV air compressor may be failing.
The air dryer
Atmospheric air contains moisture. When air is compressed and then cools, that moisture can condense inside the system.
The air dryer is designed to remove moisture before compressed air reaches reservoirs, valves and actuators.
Most systems use a desiccant cartridge to absorb moisture before periodically purging accumulated contaminants.
Without effective air treatment, moisture can contribute to corrosion, freezing and damage to pneumatic components.
Common signs that the dryer may need attention include excessive water in the air tanks, unusual purging or repeated air-system problems in cold weather.
Air reservoirs
Once treated, compressed air is stored in reservoirs, commonly referred to as air tanks.
These provide a ready supply of compressed air so the braking system does not need to wait for the compressor every time the driver presses the brake pedal.
HGVs normally use multiple reservoirs because the pneumatic system is divided into separate circuits.
This separation improves safety by helping prevent a fault in one circuit from removing pressure from the entire vehicle.
Multi-circuit protection valves
Compressed air needs to be distributed safely between different systems.
A multi-circuit protection valve divides the compressed-air supply between separate circuits while helping protect them from one another.
Depending on the vehicle, these circuits may supply:
- Front service brakes
- Rear service brakes
- Parking brakes
- Trailer braking
- Air suspension
- Auxiliary equipment
If one circuit develops a significant leak, the protection valve helps preserve pressure in the remaining circuits.
The foot brake valve and relay valves
When the driver presses the brake pedal, the foot brake valve controls the amount of braking pressure requested.
A light pedal application requests gentle braking, while a harder application requests greater braking force.
Relay valves help speed up this process.
Rather than moving large volumes of air all the way from the front of the vehicle to each brake chamber, a smaller control signal operates a relay valve positioned closer to the brakes. That valve then releases compressed air from a nearby reservoir.
This allows braking pressure to build more quickly.
Brake chambers
Brake chambers convert compressed-air pressure into mechanical force.
When air enters the chamber, it moves a diaphragm or piston, which then operates the braking mechanism.
Many heavy vehicles also use spring brake chambers on rear axles.
These contain a powerful mechanical spring used for parking and emergency braking. Compressed air is used to hold the spring off while the vehicle is moving.
This is why a vehicle may not release its parking brake until sufficient air pressure has built.
Possible brake chamber faults can include leaks, dragging brakes, uneven braking and pressure loss when the brake pedal is applied.
ABS and EBS
Modern HGV braking systems combine pneumatics with electronic control.
ABS monitors wheel speed and helps prevent wheel lock-up under braking.
EBS, or Electronic Braking System, goes further by using electronic signals, control units, sensors and modulators to manage braking pressure more precisely.
Compressed air still provides the physical braking force, but the electronic system can control it more quickly and accurately.
This can improve:
- Brake response
- Brake-force distribution
- Vehicle stability
- Trailer braking
- Fault diagnosis
Because of this, some modern air-system problems require electronic diagnostic equipment as well as mechanical inspection.
Autofast supplies a range of commercial vehicle braking components, including ABS and EBS sensors, air compressors, trailer control valves, EBS valves, brake valves, spring brakes and actuators.
Trailer air systems
On articulated vehicles, the tractor and trailer air systems work together through dedicated air connections.
This means an air-pressure fault shown on the tractor does not always originate from the tractor itself.
Possible trailer-side fault areas include:
- Air lines
- Couplings and seals
- Reservoirs
- Brake chambers
- Relay valves
- Trailer control valves
- EBS components
- Air suspension
One useful clue is whether the problem appears only after a particular trailer has been connected.
Air suspension
Compressed air is also widely used for suspension.
Air springs support the vehicle while levelling valves or electronic controls regulate ride height as the load changes.
Modern air suspension systems may also allow the chassis to be raised or lowered for loading and unloading.
If an HGV or trailer fails to rise correctly, the cause may be a general air-pressure problem or a fault limited to the suspension system.
Possible areas include:
- Air springs
- Pipework
- Height control valves
- Levelling sensors
- Solenoid valves
- ECAS components
Common HGV air-system warning signs
Some of the most common signs that an air system needs investigation include:
- Air taking longer than normal to build
- Pressure dropping while stationary
- Repeated low-air warnings
- Audible air leaks
- Pressure falling when the brake pedal is pressed
- Water regularly appearing in air tanks
- Compressor running excessively
- Air suspension failing to raise
- Brakes failing to release correctly
The behaviour of the fault can often help technicians narrow down which part of the system needs attention.
For example, pressure that falls only when the brake pedal is pressed may suggest a different problem from pressure that drops while the vehicle is simply parked.
Daily air-system checks
The air system forms an important part of the driver’s daily HGV walkaround inspection.
DVSA guidance requires drivers to check that air builds up correctly, warning systems operate and there are no obvious air leaks.
Drivers should also check that the service brakes operate correctly and inspect trailer brake-line connections for damage or leakage.
An abnormal low-air warning or significant leak should never simply be ignored because the vehicle eventually manages to rebuild pressure.
Why accurate parts identification matters
Pneumatic components can look extremely similar while having different specifications.
Differences may include:
- Pressure settings
- Port layouts
- Electrical connections
- Manufacturer specifications
- Vehicle applications
- Control characteristics
For this reason, replacement truck parts should not be identified by appearance alone.
Useful information when identifying a component includes:
- Vehicle registration
- VIN or chassis number
- Manufacturer and model
- Vehicle age
- Axle configuration
- Existing part number
- Manufacturer markings
- Clear photographs of the original component
Autofast supplies genuine, OEM and aftermarket HGV parts across braking, suspension, electrical, engine and other commercial vehicle systems.
Need help identifying an HGV air-system part?
If diagnosis has identified a faulty compressor, valve, dryer, brake chamber or other pneumatic component, Autofast can help source the correct replacement.
Our team can use vehicle information, existing part numbers and manufacturer data to identify genuine, OEM and quality aftermarket HGV parts, truck parts and lorry parts for leading commercial vehicle manufacturers.
Getting the correct component first time can help reduce unnecessary downtime and keep vehicles moving.
