An HGV that repeatedly loses air pressure should not be ignored. Compressed air is fundamental to the vehicle’s braking system and may also operate suspension and other auxiliary systems.
A small leak somewhere within a tractor or trailer can cause the compressor to work harder than normal, increase the time required to build pressure and, in more serious cases, trigger low-air warnings.
The challenge is identifying where the air is escaping.
Common causes of HGV air-pressure loss
An HGV pneumatic system contains numerous pipes, valves, reservoirs and actuators, meaning pressure can potentially be lost from many different locations.
Common areas include:
- Damaged air lines
- Loose or leaking fittings
- Trailer couplings and seals
- Brake chambers
- Air dryer assemblies
- Multi-circuit protection valves
- Relay valves
- Foot brake valves
- Air reservoirs
- Air suspension components
- Trailer braking systems
The way the pressure behaves can often provide useful clues.
Does the pressure fall while the vehicle is stationary?
If pressure drops with the vehicle parked and the brakes released, there may be a continuous leak somewhere within the pneumatic system.
Listen for escaping air around:
- Pipe connections
- Air reservoirs
- Dryer assemblies
- Suspension air springs
- Valves
- Tractor-trailer connections
DVSA guidance requires drivers to check that air builds correctly and that there are no obvious air leaks during the daily walkaround inspection.
Does pressure fall when the brake pedal is pressed?
If pressure remains relatively stable until the service brake is applied, the fault may be associated with components that are only pressurised during braking.
Possible areas include:
- Brake chambers
- Brake hoses
- Relay valves
- Foot brake valve
- Trailer braking components
A technician can use the behaviour of the system to help isolate the affected circuit.
Does the problem only happen with a trailer connected?
A tractor showing low pressure does not automatically mean the tractor is at fault.
If the system behaves normally without a trailer but pressure begins falling once a particular trailer is connected, the trailer system should form part of the investigation.
Common trailer-side leak points include:
- Red and yellow air lines
- Coupling seals
- Trailer reservoirs
- Brake chambers
- Relay valves
- EBS components
- Air suspension
Could the compressor be causing the problem?
Possibly, but slow air build-up and air loss are not the same thing.
A worn compressor may struggle to replenish the system quickly enough, but a healthy compressor can also appear ineffective if a large amount of air is escaping elsewhere.
Before replacing a compressor, the wider system should be checked for leaks and control faults.
See our guide to signs your HGV air compressor may be failing for more detail.
Could the air dryer be leaking?
Yes.
Air dryers contain valves and seals that can develop faults, while problems with the purge mechanism can result in abnormal air loss.
Moisture or contamination elsewhere in the system can also indicate that the dryer requires attention.
Commercial vehicle air-treatment systems are designed to remove moisture from compressed air before it reaches downstream components. Moisture can otherwise contribute to corrosion and freezing.
Should an HGV be driven with an air leak?
A significant or unexplained air-pressure problem affecting the braking system requires proper investigation.
Drivers should not simply allow the system to rebuild pressure and continue operating without identifying why pressure is being lost.
DVSA states that drivers are responsible for ensuring the vehicle is safe to drive and that defects identified during walkaround checks should be reported.
Need help identifying an air-system component?
If diagnosis identifies a faulty valve, brake chamber, air dryer, compressor or another pneumatic component, AutoFast can help source the correct replacement.
We supply genuine, OEM and quality aftermarket HGV parts and truck parts for leading commercial vehicle manufacturers.
Accurate identification using the vehicle details and existing component number can help prevent unnecessary downtime and ensure the correct part is supplied first time.
