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Modern automotive disk brake systems can generate extremely high temperatures under high but short time braking loads or relatively light but continuous braking. One consequence of the high disk and pad temperatures is a gradual heating of the brake hydraulic fluid which can lead to boiling of absorbed water and loss of braking. This paper describes an experiment in which fluid and braking system temperatures, pressures, and operating conditions (vehicle speed, braking energy dissipation, etc) were measured in three different classes of operating vehicles during brake application and subsequent brake release. Brake failure was observed and correlated to moisture content of the fluid, severity of brake application and application time. Temperatures predicted by a finite element analysis agree well with the measured temperatures and can be used to estimate the time at which the fluid boils.