Brake fade and falling air pressure on a long descent
You are driving a 32-tonne articulated tipper loaded with wet sand down a 6 km descent on the EN1 in Manica Province. The gradient is signed at 8%. You have a compression-release engine brake and a retarder, but you have been using the foot brake because traffic is slow. Halfway down, the brake pedal feels soft and travels closer to the floor. A low-pressure warning buzzer sounds briefly, then stops. You smell hot brake lining. The next lay-by is 800 m ahead; the road behind is clear for 300 m. Your air pressure gauge has dropped from 8.0 bar to 5.5 bar. The trailer has spring brakes that apply when pressure is lost. Your load's centre of gravity is high.
1. What is happening and what must you do immediately?
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The soft pedal, falling pressure and hot-lining smell indicate brake fade and likely air loss. Do not pump the brake. Release the foot brake, use the engine brake/retarder and select a lower gear immediately. Keep the speed low and steady. Brake gently and continuously if needed, but avoid repeated applications that add heat. Aim for the lay-by 800 m ahead; if you cannot reach it, choose the safest place to stop-hard shoulder or wide verge-before pressure falls below the spring-brake threshold, around 4.5 bar. At 5.5 bar you still have some service braking, but it is diminishing. Warn following traffic with hazard lights.
2. Why should you not use the parking brake at speed?
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The parking brake on a heavy combination usually applies the spring brakes on the drive axles or trailer. Applying it at speed can lock the rear wheels, cause a jackknife or trailer swing, and reduce steering control. It is an emergency system, not a normal retarder. Use it only after stopping, or if service brakes have failed completely and you must stop-then apply it progressively while steering straight. The retarder and engine brake are designed to control speed on a descent without overheating the foundation brakes.
3. What checks and decisions after stopping?
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Park on level ground, chock wheels, and let the system cool. Do not touch hot drums. Check air pressure build-up and listen for leaks; drain air tanks if fitted with manual drains. Inspect for glazed linings, oil contamination, and slack adjuster travel. The vehicle should not continue until pressure holds and brakes are tested. If the load is high-CG wet sand, check for load shift. Report the defect and arrange recovery or repair. Do not continue downhill just because the buzzer stopped: intermittent warnings often mean a developing fault.