What would cause hydraulic drum brakes to "fade" during application?

The fading of hydraulic drum brakes during application primarily occurs due to excessive temperature. When the brakes are applied, the friction between the brake shoes and the drum generates heat. If the temperature becomes too high, it can lead to a reduction in the braking efficiency—this is what is known as brake fade. Excessive heat can cause the brake fluid to vaporize, creating gas bubbles which compress during subsequent brake applications, reducing the hydraulic pressure and the effectiveness of braking. In addition, the drum and shoe materials can lose their friction characteristics at high temperatures, further contributing to fade.

Understanding the dynamics of braking systems is crucial since the performance of hydraulic systems directly relies on the properties of the brake fluids and materials used. While low fluid levels, leaks, and weak return springs can all contribute to brake performance issues, they do not directly precipitate the specific condition known as fading, which is centered around excessive heat accumulation during prolonged or heavy braking.