Low air pressure and brake fade on a mountain descent
At 05:40 you leave Al Hamra with a three-axle rigid truck and tandem-axle trailer, gross weight 32,400 kg, loaded with bagged cement. The route crosses the mountains and includes a signed 9 km descent at 8%, with two hairpins and no overtaking. At the top, system pressure is 7.8 bar and the low-pressure warning is silent. Two kilometres down, you smell hot brake lining, the service pedal feels firm but the truck is not slowing as expected, and the gauge falls to 5.6 bar. The compressor cuts in, but pressure keeps drifting down. Your exhaust brake is switched on, but the gearbox is in a high gear, so retardation is weak. A gravel runaway arrester bed is 600 m ahead on the right; a loaded tanker is 80 m behind. The road is dry and daylight is breaking.
1. What is the vehicle telling you, and what should you do in the next few seconds?
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The falling gauge with a running compressor points to air loss or a compressor/leak fault, not just normal consumption. The hot lining smell and poor retardation indicate brake fade, made worse by riding the service brake down an 8% grade. Immediate actions: stop accelerating, select a lower gear immediately to raise engine speed and exhaust-brake effect, and apply firm, steady service-brake pressure rather than pumping. If speed still rises and pressure approaches the warning threshold, use the arrester bed rather than continue. Warn the tanker behind with hazard lights and, if fitted, the retarder/brake lights. Do not coast or switch off the engine. Once stopped, secure the vehicle, chock wheels and treat the air system as defective until tested.
2. Why does the high gear matter, and how should this descent have been set up before leaving the top?
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An exhaust brake produces little retarding power at low engine speed; in a high gear the engine is barely turning, so the vehicle relies almost entirely on the foundation brakes. Before a signed 8% descent, the driver should stop at the top, check pressure build-up and leakage, select a gear low enough to hold a safe speed on the exhaust brake alone, and keep engine speed in its efficient braking band. The correct technique is to descend slowly in that low gear, using short, firm brake applications to trim speed and then releasing, so linings cool between applications. Continuous light pressure overheats drums and causes fade, exactly as described. The driver should also confirm trailer brakes are working and that the load does not exceed the permitted gross weight.
3. After stopping safely, what checks and decisions determine whether the truck may continue?
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With wheels chocked and the park brake applied, check for air leaks at lines, chambers, valves and drain valves, and listen for a constant hiss. Run the standard test: build pressure to governor cut-out, stop the engine, and observe pressure loss over time; then check that the low-pressure warning and spring brakes operate at the specified threshold. Inspect brake chambers, slack adjusters and pushrod travel, and feel drums for abnormal heat. A hot lining smell means the brake may be damaged even if pressure later holds. Also check the compressor drive belt and drain any moisture from reservoirs. If pressure cannot be maintained, if a chamber or line leaks, or if pushrod travel is out of adjustment, the vehicle must be repaired before continuing. Record the defect and do not let schedule pressure override the decision.