Brake Pedal Sinking on a Mountain Descent
You are descending a steep, winding mountain road in a loaded pickup with four passengers. The descent is about 6 km long. You have been using the footbrake steadily to hold a moderate speed. Two kilometres down, the pedal begins to feel soft and travels further before the vehicle slows. You smell a hot, acrid odour. The handbrake is mechanical and in good order. There is a lay-by 200 metres ahead on the right. The engine temperature gauge is normal. The road behind is clear; a car is following about 50 metres back.
1. Diagnose what is happening and explain why riding the footbrake caused it.
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The symptoms - a soft, long-travel pedal, poor braking and a hot acrid smell - point to brake fade from overheating, most likely boiled brake fluid or overheated pads/discs. Continuous light-to-moderate braking down a long descent converts the vehicle's potential energy into heat far faster than the brakes can shed it. When the fluid boils, vapour in the lines compresses instead of transmitting pressure, so the pedal sinks and braking weakens. The load of passengers and cargo increases the energy the brakes must absorb. The engine temperature being normal is a red herring: it tells you nothing about brake temperature.
2. What should you do in the next 200 metres, and what must you not do?
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Aim for the lay-by and stop there. Slow gently using the brakes you still have, and use engine braking - select a low gear so the engine holds the vehicle back - rather than relying on the pedal. Do not pump the pedal repeatedly in the hope of restoring it; if the fluid has boiled, pumping will not fix it and wastes the little braking you have. Do not switch off the engine or coast in neutral. Do not pour water on hot brakes. Warn the car behind by signalling early and, if needed, with hazard lights. Once stopped in the lay-by, apply the handbrake, let everything cool, and do not continue until you have checked the brake fluid level and the pedal feels firm again.
3. How should you have descended this road in the first place, and what would you do if no lay-by were available and the pedal went to the floor?
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The correct technique is to select a low gear at the top and let engine braking do most of the work, using the footbrake in short, firm applications rather than continuous light pressure, and keeping speed low enough that the brakes have time to cool. If the pedal went to the floor with no lay-by, use the mechanical handbrake progressively - not snatched, which can lock the rear wheels and spin the vehicle - change down through the gears to slow by engine braking, sound the horn and use hazard lights to warn others, and look for any escape route such as a gravel run-off or a rising bank. Steer for the softest available obstacle rather than continuing to gather speed.