<![CDATA[The performance of hydraulic braking systems is strongly influenced by the condition of brake fluid, particularly boiling point and tendency to form vapor bubbles that may lead to vapor lock. Glycol-based brake fluids, such as DOT 4 and 5.1, are hygroscopic, causing moisture content to increase over their service life. Therefore, this study aimed to examine the effect of moisture percentage on boiling point and bubble formation characteristics of DOT 4 and 5.1 brake fluid. An experimental method was applied by adding distilled water to brake fluid at varying concentrations of 0%, 1%, 2%, 3%, and 4%, followed by gradual heating. The boiling point was recorded using a temperature sensor connected to a data logger. At the same time, bubble formation was visually observed during the heating process using a high-speed camera with a frame rate of 240 fps. The recorded images during the analysis were subsequently examined using ImageJ processing software. The results showed that increasing moisture content led to a significant reduction in boiling point for both types of brake fluid. At identical temperature levels, fluids with higher moisture content indicated larger bubble volume formation. These results indicated that moisture content played a critical role in degrading the thermal performance of brake fluid and increased the risk of vapor lock in automotive braking systems.]]>
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