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Can I mix DOT3 and DOT4 brake fluid

The idiot light for my brake system, not the ABS, kept coming on when I stopped hard so I thought the cheapest fix to try was to add some more fluid. My owner's manual calls for DOT3 but when looking at the wide array of choices at WallyWorld I saw DOT4 had higher boil off specs than DOT3 and was about the same price, so I bought it and added it to my car. Afterward I thought I remembered something from the past about not mixing different types of brake fluids.

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So here are the questions: Did I mess up my brake system by adding synthetic DOT4 fluid to the manufacturer's original DOT3, probably non-synthetic, fluid? Do I now need to get my system flushed?

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