HGV air brake and air system faults: low pressure, leaks, dryers and what to do at the roadside

Every heavy goods vehicle depends on compressed air to release its brakes, operate its clutch or gearbox on many models, and level its suspension. When the air goes, the vehicle stops, by design. Understanding the system is the difference between a quick roadside fix and a dangerous improvisation.

How does an HGV air system work?

An engine-driven compressor pumps air through an air dryer, which removes moisture and oil vapour using a desiccant cartridge and purges itself periodically, into a wet tank and then into separate service tanks for the front and rear brake circuits, the parking circuit and auxiliaries. A governor or the dryer's unloader stops the compressor at cut-out pressure, typically in the range of eight to twelve bar depending on the vehicle, and restarts it at cut-in. The foot valve meters air to the brake chambers when the driver brakes. The parking brake uses spring brake chambers: powerful springs apply the brakes mechanically, and air pressure is used to hold them off. Trailers receive supply air through the red coupling and service air through the yellow coupling, with the ISO 7638 electrical connector carrying the ABS or EBS signals.

A dashboard gauge or display shows the circuit pressures. A low-pressure warning lamp and buzzer operate when pressure falls below a set point, and if pressure keeps falling the spring brakes apply automatically. The warning exists so that the driver stops in a safe place before the vehicle stops itself in an unsafe one.

What does the low-pressure warning mean?

It means the system is losing air faster than the compressor can replace it, or the compressor is not replacing it at all. On seeing it, indicate, pull into a safe place while you still have braking, stop and apply the parking brake. Do not try to reach the depot. If the warning appears while the vehicle is stationary with the engine running, the compressor or governor side is the suspect; if it appears while driving after a normal build-up, a leak or a failed valve is more likely.

Common air system faults

Fault	Typical symptoms
Air leak at a brake chamber diaphragm, line or fitting	Audible hiss, pressure drops with the engine off, slower build-up
Leak at the foot valve or a relay valve	Hiss when the brakes are applied, or continuous leak from the exhaust port
Saturated or failed air dryer	Water in the tanks when drained, frozen lines in winter, compressor cycling constantly
Worn compressor	Long build-up from cold, oil in the system, low cut-out pressure
Faulty governor or unloader	Compressor never cuts out, or cuts out too early
Trailer coupling or line fault	Trailer brakes drag or will not release, hiss at the couplings, ABS or EBS warning
Seized or over-adjusted slack adjuster	Brake drag, heat and smell from one wheel, tyre damage
Failed pressure sensor or switch	Warning with normal gauge readings, or no warning with low pressure

How to find a leak safely

With the vehicle stopped on level ground, engine running until cut-out, then engine off: listen at the wheels, the chambers, the couplings and the valves with the brakes released, then with the footbrake applied and held by a second person or a suitable weight. A leak that appears only with the brakes applied is on the service side; a continuous leak is on the supply side. Watch the gauge: a slow but steady drop with the engine off confirms a leak. Never crawl under a vehicle on air suspension that is not supported, never put your hand near a chamber pushrod, and never disconnect a line under pressure.

What not to do

Do not wind off, cage or release a spring brake to move a vehicle whose brakes have applied through air loss, other than under the controlled procedure in the handbook for recovery purposes with the vehicle secured. Do not block a leaking line, tape a coupling or bypass a valve to get home. Do not keep driving with the warning on, and do not drive a trailer with a dragging brake to the next stop; the heat can destroy the wheel end and start a fire.

Air dryers and winter

The dryer cartridge is a service item, usually replaced annually or at a mileage interval. A saturated dryer passes water into the tanks, which corrodes valves and, in winter, freezes in lines and valves so that brakes stick on or will not apply. Drain the tanks regularly if the vehicle has manual drain valves, replace the cartridge on schedule and treat repeated compressor cycling as a dryer or leak fault.

Trailer air faults

Before blaming the trailer, check the couplings: the red supply line must be connected and fully seated for the trailer brakes to release, the yellow line for them to apply under control, and crossed or partially seated couplings are a frequent cause of trailer brakes that drag or do not work. Check the ISO 7638 plug for damage or a poor seat when an ABS or EBS lamp is lit; the trailer's braking electronics are powered through it. A trailer that will not release after coupling may also have a failed emergency relay valve or a park valve left applied.

What does DVSA do about air and brake defects?

The Categorisation of Vehicle Defects treats an air leak that affects braking performance, a spring brake that does not function, a brake chamber pushrod at the limit of travel, a seized slack adjuster and inoperative trailer braking as prohibitable defects, most of them immediate. A brake defect discovered at a breakdown is likely to be checked against the maintenance history, and a defect that a walkaround or inspection should have found may be S-marked. Air build-up time and any warning during the walkaround should be reported, not driven with.

Roadside repair or recovery?

A mobile HGV mechanic can usually replace a brake chamber, a line, a fitting or a coupling, change a dryer cartridge, free a slack adjuster and cure many leaks at the roadside. Compressor failure, foot valve or relay valve failure, multiple leaks, a spring brake that has applied because of a structural failure, and any defect the mechanic cannot make safe should mean controlled recovery to a workshop, with the vehicle moved only under the recovery operator's procedure.

What to tell the provider

Registration, make and model; what the gauge shows and whether the warning is on; whether the leak is continuous or only when braking; where the hiss seems to be; whether the vehicle is a rigid or a tractor and trailer, and whether the trailer can be uncoupled; whether the vehicle is on air suspension and has dropped; and the exact, safely accessible location. Ask the provider whether they carry chambers, lines, fittings and dryer cartridges for your make.

HGV247 lists mobile mechanics and brake specialists with their published capabilities. It is a directory, not a repairer, and nothing here replaces your employer's procedure or the vehicle handbook. In danger, call 999.