When an HGV takes noticeably longer than usual to build air pressure, something within the pneumatic system may no longer be operating correctly.
The compressor is an obvious suspect, but it is far from the only possible cause.
A leak, restricted air supply, faulty dryer, control-valve problem or trailer defect can all result in slow pressure build-up.
What does the compressor do?
The compressor draws in atmospheric air, compresses it and supplies it to the vehicle’s air-treatment system.
That compressed air is then dried and distributed to the vehicle’s reservoirs.
As pressure falls through normal use, the compressor replenishes the system.
If the compressor can no longer supply air efficiently, the vehicle may take longer to reach its normal operating pressure.
Common causes of slow air-pressure build-up
Potential causes include:
- Worn air compressor
- Compressor intake restriction
- Significant air leak
- Air dryer fault
- Purge valve problem
- Governor or unloader fault
- Non-return valve problem
- Multi-circuit protection valve fault
- Trailer air leak
- Excessive pneumatic demand
Because several faults can create the same symptom, replacing the compressor without further diagnosis may not solve the problem.
Check for air leaks first
A compressor may be working normally but effectively trying to fill a system that is leaking continuously.
If the vehicle takes longer to build air than it used to, listen for escaping air while pressure is building.
Potential leak points include:
- Brake chambers
- Air lines
- Couplings
- Reservoirs
- Air dryer
- Valves
- Suspension air springs
DVSA specifically requires drivers to check that air builds correctly and that there are no air leaks during the daily walkaround inspection.
Could the air dryer restrict pressure build-up?
An air dryer sits between the compressor and the rest of the pneumatic system.
Its job is to remove moisture and contamination before air reaches the reservoirs.
Faults within the dryer, purge mechanism or associated air-processing unit can interfere with the normal supply of compressed air.
Modern air-processing systems can combine the dryer and multi-circuit protection functions into a single unit, making accurate diagnosis increasingly important.
Does the problem change when the trailer is disconnected?
This can be a useful diagnostic clue.
If the tractor builds pressure normally on its own but becomes noticeably slower once a trailer is connected, the additional air loss or demand may be coming from the trailer.
Possible causes include leaking:
- Trailer air lines
- Couplings
- Brake chambers
- Reservoirs
- Valves
- Suspension components
- Signs the compressor itself may need investigation
Possible compressor-related symptoms include:
- Increasingly slow pressure build-up
- Difficulty reaching normal system pressure
- Excessive compressor operation
- Unusual mechanical noise
- Oil contamination downstream
- High compressor temperatures
None of these symptoms alone proves that the compressor needs replacing, but together they can justify further investigation.
Don’t ignore gradual changes
Slow air build-up often develops gradually.
Drivers who use the same vehicle every day may be the first to notice that the truck takes longer to reach operating pressure than it previously did.
Reporting that change early can allow a workshop to investigate before it develops into a more serious fault or vehicle downtime.
Need replacement truck air-system parts?
If diagnosis identifies a faulty compressor, dryer, valve or another component, Autofast can help identify and source the correct truck parts and HGV parts using vehicle details, manufacturer information and existing component numbers.
Getting the right part first time can reduce unnecessary vehicle downtime and avoid replacing components that were not responsible for the original problem.
