When an articulated HGV begins losing air pressure, one of the first questions is whether the problem originates on the tractor or trailer.
Because the two systems are connected, a trailer leak can show up as falling pressure on the tractor’s gauges.
Understanding this relationship can make diagnosis considerably easier. Our guide to why an HGV loses air pressure covers the wider fault list.
How are the tractor and trailer connected?
The tractor supplies compressed air and braking control to the trailer through dedicated connections.
The trailer then has its own:
- Pipework
- Reservoirs
- Brake chambers
- Valves
- EBS components
- Suspension system
A fault anywhere within that trailer system can therefore consume air supplied by the tractor.
Does the tractor hold pressure on its own?
This is a useful diagnostic distinction.
If the tractor maintains pressure normally when operating without a trailer, but begins losing pressure once a specific trailer is connected, the trailer should form part of the investigation.
That does not prove the trailer is at fault, but it considerably narrows the search.
Common tractor-side leak points
Possible tractor-side causes include:
- Air compressor
- Air dryer
- Air reservoirs
- Protection valves
- Brake chambers
- Pipework
- Foot brake valve
- Tractor control valves
- Suspension components
Common trailer-side leak points
Possible trailer faults include:
- Red and yellow air lines
- Couplings and seals
- Trailer reservoirs
- Brake chambers
- Relay valves
- EBS modulator
- Trailer control valves
- Air springs
- Suspension valves
DVSA specifically requires drivers to check that trailer brake-line couplings are correctly connected and free from leaks, damage and wear.
Does the leak appear only when braking?
If the trailer holds pressure until the service brake is applied, components that receive pressure during braking may need investigation.
These can include:
- Brake chambers
- Relay valves
- Brake pipework
- Trailer braking valves
Does the trailer suspension drop?
A trailer that gradually sinks may have a suspension-side air leak rather than a brake-system leak. See HGV air suspension not rising if ride height is the main symptom.
Possible causes include:
- Damaged air spring
- Height-control valve
- Air line
- Connector
- ECAS component
Don’t rely on sound alone
Large leaks may be obvious, but small or intermittent faults can be difficult to locate by listening alone.
Electronic braking systems can also record faults that help technicians narrow down the affected circuit or component.
Need tractor or trailer air-system parts?
AutoFast supplies lorry parts and trailer components across braking, suspension and the pneumatic system.
Whether the fault is on the tractor or trailer, supplying the vehicle or trailer details and existing component number can help identify the correct truck parts replacement quickly.
