Brake fluid is primarily composed of glycol ether or mineral oil, depending on whether it is a glycol-based fluid (like DOT 3, DOT 4) or a mineral oil-based fluid (like DOT 5).
Out of the options you provided:
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Is mostly just engine oil: This is not correct. Brake fluid is not made from engine oil.
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Is always blue: This is not correct. Brake fluids can come in various colors, including clear, amber, or even blue, depending on the type and brand, but it's not always blue.
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Has a high boiling point: This is correct. Brake fluid is designed to have a high boiling point to withstand the heat generated during braking without boiling, which could lead to brake failure.
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Lubricates transmission parts: This is not correct. Brake fluid is specifically designed for braking systems and is not used for lubricating transmission parts.
Therefore, the correct statement is that brake fluid has a high boiling point.