Low Air Warning on a 6x2 Tractor Unit

The proposal does not follow the evidence. The main defect is a pressure loss, not necessarily a compressor fault. The leak appears only when the park brake is released. Releasing the park brake supplies air to the spring-brake chambers to compress the springs; applying the park brake exhausts those chambers. A leak that stops when the park brake is applied therefore points to the spring-brake circuit: a chamber diaphragm, a chamber seal, a hose, or a relay valve feeding the rear axle. The audible hiss near the rear axle supports this.

Raising governor cut-out to 8.5 bar would raise system pressure and may make the warning less frequent, but it does not seal the leak. It also increases stress on chambers, valves and pipework, can worsen water carry-over, and may take the system outside the manufacturer's specification. Replacing the compressor is not justified by a build time that may still be within tolerance; the compressor is producing pressure, but the system is losing it. The water in the drains also suggests the air dryer or its cartridge needs attention, which is a separate issue.

A correct judgement is: repair the leaking spring-brake circuit first, restore correct governor settings, service the dryer and drains, then retest build time and leak rate.

1. Make safe: chock the wheels, lower the vehicle if lifted, and treat spring brakes as stored energy. Do not go under an unsupported vehicle.
2. Stabilise the system: run the engine to normal cut-out, stop the engine, and allow pressure to settle. Record cut-out and cut-in pressures.
3. Leak test in the fault condition: with the park brake released and service brakes off, apply soap solution to the rear spring-brake chambers, chamber clamps, hoses, relay valve, pipe unions and fittings. A hiss alone is not enough; confirm the exact source.
4. Compare conditions: apply the park brake and repeat. If the leak stops, the spring-brake release circuit is confirmed. Also apply the service brake and check for a separate service-brake leak.
5. Isolate circuits: use the reservoir drain valves or manufacturer-approved isolation points to identify which reservoir feeds the leak. Check check valves and the four-circuit protection valve for cross-leakage.
6. Check air quality: drain all reservoirs and inspect for water or oil. If water is present, test the air dryer, purge valve and cartridge.
7. Recheck performance: after repair, measure build time, governor cut-in/cut-out, and leak rate against the manufacturer's specification. Do not road-test until the static tests pass.

Release only when the repair is proven, not when the warning lamp is merely reset. The criteria should include:

• The leaking component is replaced with the correct rated part, not sealed or bypassed.
• Static leak rate is within the manufacturer's limit with the park brake released and with service brakes applied.
• Compressor build time from 0 to cut-out is within specification, and governor cut-in and cut-out are correct and unaltered.
• Spring brakes apply and release fully; there is no dragging or delayed release.
• Low-pressure warning and protection valve function are correct.
• Reservoirs are drained, the air dryer is serviced if water was found, and no oil contamination remains.
• A road test confirms brake performance and no low-pressure warning under load.

The job card should record the fault found, test pressures before and after repair, parts fitted, the leak-test result, and the name of the person who released the vehicle. If the vehicle is needed that afternoon, the correct decision is to delay it until these checks are complete; a pressure loss on a gradient is a safety-critical defect.