Low air pressure on a 6% descent
At 14:20 a driver takes a 3-axle tipper, plated at 32,000 kg and loaded with 24 tonnes of aggregate, onto a mountain road south of Taif. The descent is 6 km long at an average 6% grade, posted 60 km/h for heavy vehicles. About 2 km down, in 8th gear, the driver has been holding the service brake on continuously to keep the speed at 55 km/h. The low-pressure warning buzzer sounds and the gauge shows 5.5 bar and falling; normal governor cut-out is 8.1 bar and cut-in is 7.0 bar. The trailer brakes were adjusted two days ago, but the driver noticed a hiss near the mid-axle area at the last stop. Brake drums smell hot. The next lay-by is 400 m ahead; the bottom of the grade is 4 km away. Traffic behind includes two cars.
1. What is happening to the braking system, and what should the driver do in the next few seconds? Give the order of actions.
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Two things are happening at once. The hot drums mean the service brakes are fading: continuous drag has overheated the linings and drums, so friction is falling. At the same time the gauge is falling toward the spring-brake application range (roughly 2-2.5 bar), which fits the hiss he heard earlier - the compressor cannot keep up with a leak plus repeated applications.
Order of actions:
- Release the service brake and select a lower gear now (low range, around 3rd/4th) so the engine holds the speed. Do not pump or fan the brake.
- Keep the steering steady, warn following traffic, and aim for the lay-by 400 m ahead.
- If pressure is still falling when he reaches it, stop deliberately by pulling the park control, which exhausts air and applies the spring brakes, rather than waiting for them to apply themselves. Hold the wheel straight and expect the combination to jerk.
- Chock the wheels and do not continue to the bottom.
If the vehicle will not slow and a runaway develops, use the escape bed or berm, not the downhill lane.
2. Explain why holding the service brake for 2 km created this situation, and what the driver should have done before starting the descent.
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Continuous light brake use on a long grade converts speed into heat faster than the drums can shed it. As drums heat they expand and lining friction drops, so the driver needs more pedal for the same retardation, which feeds more heat - classic fade. Every application also consumes air, and with a leak the compressor cannot recover to the 8.1 bar cut-out.
The correct technique before the descent is to read the grade sign, select a gear that would climb the hill (roughly 3rd/4th low range for 24 t on 6%), and let the engine and any retarder or exhaust brake do most of the work. Use short, firm brake applications to drop 10-15 km/h, then release so the drums cool. Check the gauge at the top, do a low-speed brake test, and confirm any leak is fixed. The lay-by 400 m ahead is only useful if the brakes still work when he gets there, which is why the gear change and the leak matter more than reaching the lay-by itself.