Brake fade on a loaded six-kilometre descent
You are driving a pick-up down a 6 km descent in the West Region, carrying about 700 kg of cement bags in the load bed and two passengers in the cab. The road is tarmac with broken patches in the bends. Two kilometres into the descent you notice a hot, acrid smell, and the brake pedal now travels almost to the floor before it bites. The vehicle pulls to the right under braking, and the pedal feels spongy rather than firm. You are in third gear, road speed has risen past 70 km/h even though you are not pressing the accelerator, and the next bend tightens sharply to the left with an unguarded edge. In the mirror you can see a light haze behind the right rear wheel. The load is not strapped down and shifts when you change direction. There is a straight, rising verge on your right about 60 metres ahead. The brakes were last looked at before the rainy season.
1. Diagnose what has happened and set out, in order, what you do in the next 200 metres.
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Diagnosis: brake fade from sustained use. Continuous braking on a descent converts speed into heat faster than the drums and pads can shed it. The pads glaze, and the fluid - which absorbs moisture over time and therefore boils at a lower temperature - vaporises in the line. Vapour compresses, so the pedal goes long and spongy and braking force collapses. The pull to the right plus the haze on the right rear wheel suggests that side is hotter still, or that a wheel cylinder is leaking fluid onto the lining.
Actions, in order:
- Get off the pedal. Continuous pressure adds heat and buys almost nothing.
- Change down to second now. Engine braking is unaffected by brake temperature - it is the only system still working properly.
- Do not switch the engine off. You would lose engine braking, power steering assistance and the brake servo, and risk the steering lock.
- Apply the brakes in short, firm applications rather than riding them.
- Use the 60 metres of rising verge: indicate right, cross it and let the gradient and the ground stop you.
- Once stopped, chock the wheels and let everything cool. Do not pour cold water on hot drums and do not open the fluid reservoir. Call for recovery; do not continue the descent.
2. Suppose the pedal now reaches the floor and the speed is still climbing. Rank your options and justify the one you reject.
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In order of preference:
- Run-off onto the rising verge or bank. It converts kinetic energy into a controlled, low-speed impact on your terms, at a place and time you have chosen. That is the whole point of an escape route.
- Gear down further and accept the over-revving. A rebuilt engine is cheap compared with a vehicle over an unguarded edge, and the drivetrain will still retard you.
- Handbrake, applied progressively, never snatched. It is mechanical and may still work if the hydraulics have failed. Snatching it locks the rear wheels and, especially in a bend, spins you.
- Rejected - switching off the engine. It removes engine braking, power steering assist and the brake servo, and can lock the steering column. It makes a bad situation unrecoverable.
- Rejected - crossing into the oncoming lane. That is a blind left-hand bend: you would be swapping a known hazard for an unseen one.
Throughout: hazard lights on, warn the passengers, and use the horn on the approach to the bend so anyone coming up knows something is wrong.
3. Which pre-departure decisions contributed to this situation, and what would you check before any loaded descent?
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Contributing decisions: loading 700 kg without checking the brakes first; leaving the load unstrapped, so it shifts and adds a steering and stability problem on top of a braking one; descending in third and controlling speed with the brakes instead of with the gearbox; and running on brake fluid of unknown age - hygroscopic fluid past two years old boils much earlier than fresh fluid.
Before a loaded descent, check: fluid level and age, pad and lining thickness, no weeping at the wheel cylinders, handbrake adjustment and travel, tyre pressures and condition for the load, and the load itself strapped hard against the cab so it cannot shift. Then plan the descent before starting it: come down in the same gear you would need to climb it, and use the brakes in short applications only to trim speed before a bend, never to hold it for kilometres. On this road that is second gear, roughly 30 km/h, with the brakes cold until they are needed.