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Messrs OldDuke and Porkface,
Brake fluid DOT 3 and 4 are a glycol-based anhydrous hydraulic brake fluid. Dot 4 may have some other compound to raise the boiling point. Some sort of ester.
As for synthetic fluid, well, I can think of Dot 5 fluid, and that is silicon, and silicon is not compatible with the glycol-based anhydrous hydraulic brake fluid. To change to silicon, you have to flush the glycol brake fluid completely.
http://en.wikipedia.org/wiki/Brake_fluid
The fluid should have a clear, if pale, honey color.
Change the glycol brake fluid every two to three years. As is goes darker in color, indicating brake fluid brake down. The Motive power bleeder helps.
Hope that helps you,
Duff.
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