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tjl said: Would it be safe to assume that an opened and then recapped container of brake fluid will absorb moisture at about the same rate as the brake fluid that is in the car's reservoir?
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__. Well, no. The reservoir in your car is (supposed to be) sealed with a deformable gasket. As fluid is pumped to the wheel cylinders (calipers), the gasket should drop down with the fluid, thus keeping air away from the fluid. Of course, this will not be perfect but it's better than entire empty portion of a can. In real life, it works pretty well at keeping air/water vapor away from the fluid. It's also why your reservoir cap has all the warnings about cleaning the cap before you remove it, keeping the seal, and generally being careful with it.