How does water get into brake fluid when its a closed system?
 

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One source is the air gap in the brake fluid reservoir. Most brake fluids are hydroscopic so it absorbs water vapor from the air gap.

And through microscopic pores in the seals and hoses. Most brake fluid reservoirs are transparent so you can check the level without opening it and thus contaminating the fluid. In a year's time brake fluid can contain as much as 2% water.