What is brake fade, and what causes it?
Brake fade is a loss of braking effectiveness from heat — the pedal may still feel firm, but the car does not slow as it should. It is the reason repeated hard braking on a long descent is dangerous in a way that a single hard stop is not.
What brakes actually do. A brake converts the vehicle's kinetic energy into heat, through friction between pad and disc. That heat then has to be dissipated into the air. Braking hard repeatedly generates heat faster than it can be shed, and temperatures climb.
The types of fade, which have different causes and different symptoms:
Friction (pad) fade. The pad material exceeds its designed temperature range and its coefficient of friction falls. The pedal still feels normal and firm, which is what makes it alarming — you press as usual and less happens. This is the most common type.
Green fade, a related phenomenon in new pads that have not been bedded in, as binding resins outgas.
Fluid fade (vapour lock). The brake fluid itself boils. Fluid is incompressible; vapour is not, so the pedal goes soft and travels toward the floor. A long, spongy pedal after heavy use is this.
Brake fluid is hygroscopic — it absorbs moisture from the air over time, and water lowers the boiling point substantially. This is precisely why brake fluid has a replacement interval that is about age rather than mileage, and why skipping it matters more than it appears to.
Mechanical fade, where drums expand away from the shoes — largely historical on cars but relevant to older vehicles and trailers.
What causes it in practice: long descents ridden on the brakes; towing, which adds mass without adding brakes on the tow vehicle; track use; worn or contaminated pads; old brake fluid; and heavy vehicles on repeated stops.
How to avoid it:
Use engine braking on descents — select a lower gear and let the engine hold the speed, rather than braking continuously.
Brake firmly and briefly, then release, rather than dragging.
Change brake fluid at the specified interval, and consider a higher specification if towing.
Bed in new pads properly.
If fade occurs: slow using gears, use the terrain, and stop to let the brakes cool — do not apply the handbrake hard on hot discs while parked.