Air Brake Pressure Loss on a Descent

The gauge falling from 850 to 620 kPa while braking repeatedly points to air consumption/leakage outpacing compressor replenishment, not a single catastrophic failure. At a steady 7% grade, a 25.4 t vehicle needs a large continuous retarding force; short 4–5 s foot-brake applications repeated over 90 seconds cycle the relay valves many times, and each cycle vents air. If the compressor is weak, the governor cut-in is late, or there is a slow leak in a line, chamber, or dryer, reservoir pressure drops faster than it is restored.

The longer pedal travel is the key symptom. In an air system, a falling reservoir pressure means less air volume is available to move the slack adjusters and pushrods. The treadle valve must be pressed further to admit enough air to generate the same chamber pressure, so the pedal feels "long" and may eventually go to the floor with reduced retardation. This is a developing brake fade/loss-of-air condition, not normal feel.

A pressure of 620 kPa may still be above the low-pressure warning threshold on some vehicles, which explains why the buzzer has not sounded - the absence of a warning is not evidence the system is healthy. The correct judgement is to treat the falling gauge and changing pedal as an imminent failure and act before pressure reaches the warning/low-pressure protection range.

Act now, not at 2.5 km. The 400 m pull-off is the correct target. Reasoning:

• Rate of loss: Pressure fell ~230 kPa in 90 s, roughly 2.5 kPa/s under current braking. Continuing another 2.5 km at 80 km/h takes about 110 s - enough to lose another ~270 kPa, putting the system near or below warning pressure on a grade with bends. That is an unacceptable margin.
• No engine brake: The exhaust brake is unavailable, so the only auxiliary retardation is gone. There is no fallback if service brakes degrade further.
• Bends are unmarked: Braking demand may suddenly increase mid-descent, exactly when pressure is lowest.
• Light traffic: A controlled stop into a pull-off is low-risk now; a loss of braking on a bend at 80 km/h is high-risk.

Action sequence: reduce speed progressively using the remaining brakes in a smooth, sustained application rather than pumping; select a lower gear to gain engine braking; indicate early and move into the 400 m pull-off; stop, apply the park brake, chock if needed, and do not continue. Report the defect and arrange recovery or repair.

Why not the 2.5 km bay? It is only justified if the vehicle can maintain pressure and speed safely. Here the trend is downward and the pedal is already changing, so the nearer refuge dominates. Choosing the farther bay would be gambling on a system already showing failure signs.

Before the descent:

• The exhaust brake fault should have been rectified before the trip; a heavy vehicle on a 6 km 7% grade without auxiliary retardation is not fit for that route. If it could not be fixed, an alternative route or a load reduction should have been considered.
• A pre-start check should have included a compressor build-up and leak-down test: with the engine running, note how quickly pressure recovers from cut-in to cut-out, then shut down and watch for pressure drop with brakes applied. A slow leak or weak compressor would have shown up.
• Descent technique should have been low gear and intermittent, firm applications with full release between them, allowing the compressor to recover, rather than repeated short holds that heat brakes and drain air.
• The gauge should have been monitored from the top of the grade, establishing a baseline.

Once stopped - obligations:

1. Treat it as a major defect affecting the braking system. The vehicle must not be driven until repaired and the repair verified.
2. Record the defect in the daily defect/maintenance record: gauge readings, pedal behaviour, time, location, and action taken.
3. Notify the operator/manager promptly so the vehicle is removed from service and the schedule is adjusted.
4. Preserve the fault-code history for the exhaust brake, since it is relevant to the vehicle's suitability.

Under chain-of-responsibility principles, the driver must not sign off a vehicle as roadworthy when a known braking defect exists, and the operator must not pressure a driver to continue. Continuing would breach both the driver's duty and the operator's duty to ensure a roadworthy vehicle.