What brake fade actually is
Where the heat goes, and the two separate mechanisms that cause fade.
Brake fade is what happens when a braking system gets too hot to keep doing its job properly, and it can happen through either of two separate routes, one where the friction material itself starts gripping less well, and another where the fluid carrying the driver's pedal pressure starts behaving like a gas instead of a liquid.
Brake discs glowing orange under hard use are the most visible reminder that stopping a car means turning its motion into heat and then somehow getting rid of that heat again before it causes a problem.
Two ways hot brakes stop working
Every time a car slows down, the kinetic energy it had while moving turns into heat where the brake pad rubs the disc, and that heat has to be carried away fast enough that the brakes are still cool enough to work properly the next time they are needed, which is why many discs are vented with internal passages that pump air through the disc itself and shed heat faster than a plain solid disc could.
Pad fade happens when the friction material itself gets so hot that its molecular structure starts to change, and the coefficient of friction between pad and disc drops at high temperature, so the same pedal pressure produces less stopping force than it did when the brakes were cool. Fluid fade is a different failure entirely, and it happens inside the brake lines, where the hydraulic fluid that transmits the driver's foot pressure through to the caliper can absorb enough heat from the surrounding components to reach its boiling point and start forming pockets of gas inside a system designed only ever to carry liquid. The two failures can even happen on the same car during the same session without sharing a cause, since a disc can run hot enough locally to fade its pads while the fluid sitting further away in the lines never gets close to boiling, or the reverse, with fluid trapped near a hot caliper boiling well before the pad material itself has lost much grip.
A drink bottle with air trapped in it
Squeezing a plastic drink bottle that is completely full of water transmits the push almost perfectly, since water barely compresses at all, and pressing on one side of the bottle is felt immediately pushing back everywhere else inside it. Squeeze the same bottle when it has a pocket of trapped air near the top instead, and the feel changes completely, because the first part of the squeeze goes into compressing that air pocket, and the bottle feels soft and spongy for a moment before anything else happens. A brake pedal connected to fluid with gas bubbles in it behaves exactly the same way, since pressing the pedal spends its first, most urgent travel compressing those bubbles, and the pedal can sink disconcertingly close to the floor before any real braking force finally arrives. Bleeding the brakes, pushing fresh fluid through the lines to force any trapped bubbles out through a valve at the caliper, amounts to draining the bottle of its trapped air pocket and refilling it completely, restoring the direct, incompressible push the system depends on.
Different fixes for each kind of fade
Treating these as two separate mechanisms matters because they are fought with entirely different tools. Pad fade is addressed by choosing a friction material rated to hold its coefficient of friction at higher temperatures, and by getting more cooling air to the disc itself so it never reaches the temperature where the pad's performance starts to fall away. Fluid fade is addressed separately, by using a brake fluid with a higher boiling point than the fluid supplied as standard, and by keeping that fluid fresh, since brake fluid slowly absorbs moisture from the air over time and absorbed water lowers its boiling point. The standard for one common grade of fluid requires it to boil no lower than 230 °C when fresh from a sealed container, but only 155 °C once it has soaked up a few percent of water, roughly a third lower.
A team that only thinks about brake fade as one problem tends to fix the wrong half of it, fitting better pads while leaving old, moisture-laden fluid in the lines, or bleeding fresh fluid through the system while leaving cooling ducts blocked or undersized. Both failures feel similar from the driver's seat, a pedal that goes soft or a car that simply refuses to slow down as hard as it did on an earlier lap, so the way to stop it recurring on the next long run is to test each system on its own terms instead of guessing from the symptom. A car returning from a session with a soft pedal that firms back up once cooled down almost always points to fluid rather than pads, since pad performance recovers with cooling but rarely explains a pedal sinking toward the floor, while a pedal that stays firm throughout but simply needs more and more travel and effort for the same stop is a clearer sign of the friction material itself losing its edge.