Dead Sea Descent: Falling Air Pressure
You are driving a 40-tonne articulated tanker down the 8 km descent on Route 65 toward Sweimeh in 38°C heat. The vehicle has an engine retarder, ABS and a working low-pressure warning; the manual states the warning sounds at 5.5 bar and the spring brakes apply below 4.5 bar. At the top you select a low gear, but traffic keeps slowing you and you use the foot brake repeatedly. Halfway down the primary air gauge falls from 8.2 bar to 5.9 bar, the buzzer sounds, and the pedal feels soft. You smell hot brake lining and your speed has crept from 40 km/h to 65 km/h. The next pull-off is 600 m ahead; a signed escape ramp is 1.2 km ahead. The trailer carries 28,000 litres of diesel.
1. What is happening, and why is continuous light braking on this grade making it worse?
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Repeated service-brake applications turn the vehicle’s speed into heat faster than the drums can shed it. The drums and linings overheat, friction falls, and the driver gets brake fade - the pedal may feel soft while stopping force drops. At the same time, each application uses compressed air, and the compressor cannot recharge the tanks as fast as the brake chambers and valves are consuming air. The falling gauge and buzzer show that stored air is being depleted. A soft pedal can also indicate a leak.
The diesel load raises the consequence of any loss of control. The correct technique on a long descent is to use engine braking and the retarder, with short, firm brake applications to control speed, not a light continuous drag that overheats the drums and drains air.
2. Set out your immediate actions in order, using the 600 m pull-off if possible.
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- Keep both hands on the wheel and switch on hazard lights.
- Release the foot brake so the drums can begin cooling and air consumption stops.
- Downshift progressively to use engine braking; select the retarder or engine brake to its highest safe setting.
- Use one firm brake application to bring speed down, then release. Avoid pumping the pedal repeatedly.
- Aim for the 600 m pull-off. If speed cannot be controlled, or pressure keeps falling toward the 4.5 bar spring-brake threshold, use the signed escape ramp rather than passing it.
- Do not coast in neutral and do not suddenly apply the parking brake at speed, which can lock wheels or cause a jackknife.
Once stopped, chock the wheels, keep the engine running to build air if it is safe, and set the parking brake. Expect smoke and heat; keep clear of the drums.
3. After stopping, what checks decide whether you can continue, and what would make you refuse?
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Check that pressure builds to governor cut-out and holds with the engine off; listen for leaks; confirm the low-pressure warning works; test that the trailer brakes apply and release; look for glowing, smoking or cracked drums; check for a strong burnt-lining smell; and confirm wheels turn freely. Check ABS for a fault code.
Refuse to continue if pressure will not recover, if there is an audible leak, if a drum is very hot, if braking is uneven, or if the trailer brakes do not apply fully. Cool drums naturally - never pour water on hot drums. Call a fitter or heavy recovery. A low-speed test in a safe area does not excuse a known leak or overheated brake.