Brake Pedal Going Soft on a Long Descent
You are driving a loaded ten-seater van down a long, steep mountain descent on a two-lane road; the grade runs at about 8% for 6 kilometres. Three children are among the passengers. It is mid-afternoon and dry. Halfway down, you notice the brake pedal now travels noticeably closer to the floor than it did at the top and feels soft and slightly spongy. There is a faint hot, burning smell, and the van keeps picking up speed even though you are holding steady pressure on the pedal. A paved pull-out is about 400 metres ahead. Behind you, a car is following at a moderate distance.
1. What is most likely happening, and why is it dangerous right now?
Read the worked answer
This is brake fade, with probable boiling of the brake fluid.
Mechanism: a loaded van on an 8% grade for 6 km converts a large amount of potential energy into heat in the drums or rotors and pads. Continuous light-to-moderate pedal pressure gives the brakes no time to cool. Two things then happen:
- The friction material overheats and its coefficient of friction drops - the pedal feels normal but the van does not slow.
- The fluid in the calipers/wheel cylinders boils; vapour compresses easily, so the pedal goes long and spongy and eventually can go to the floor.
The burning smell corroborates overheated friction material.
Why it is dangerous now: braking capacity is degrading on the section where you need most of it, with children on board and a car close behind. If the fluid fully vaporises you can lose the brakes completely, and there is no run-off area on a mountain grade.
2. What do you do in the next 400 metres, and what do you do once you reach the pull-out?
Read the worked answer
In the next 400 metres:
- Get off the brakes. Select a lower gear now so engine braking holds the van back - that is your remaining reliable retardation.
- Use the brakes in short, firm applications to bring speed down in steps, then release, rather than riding them. Continuous light pressure is what cooked them.
- Switch on hazard lights. Keep the engine running - you need power steering and any vacuum/hydraulic assist, and you must not lose it mid-corner.
- Keep a straight, steady line, warn the following car with the hazards, and do not attempt to overtake anything.
- If the pedal goes to the floor, use engine braking plus the handbrake progressively, and look for an escape - a soft shoulder or an uphill turnout - rather than a downhill run.
At the pull-out: stop, chock the wheels, hazards on. Let the brakes cool for at least 30–45 minutes. Do not pour water on hot drums or rotors - thermal shock can crack or warp them. Do not open the fluid reservoir while it is hot. Check the fluid level and look for leaks before continuing, and continue in low gear.
3. How should this descent have been driven from the top, and what checks belong in the pre-trip?
Read the worked answer
Driving the descent: select the low gear before the grade starts - a useful rule of thumb is to descend in roughly the same gear you would need to climb the same hill. Keep the speed low enough that the brakes are needed only occasionally, in short firm applications. Speed that builds up is far harder to remove than speed that is never allowed to build.
Pre-trip checks that would have caught this:
- Brake fluid level, colour and age (fluid absorbs moisture over time and boils at a lower temperature).
- Pad/shoe thickness and any leaking wheel cylinder or caliper.
- A static pedal test: firm, high pedal that does not sink under sustained pressure.
- Load within the van's rated capacity - an overloaded van on a steep grade massively increases brake heat.
- Tyre condition and pressures, coolant level, and a check that the handbrake/parking brake holds.
Planning point: on an unfamiliar mountain route, ask about grade and length before you start, and build in a stop at the top to check brakes and select the gear.