Brake fluid absorbs water (hygroscopic), and should be replaced as per the manual instructions, typically every 2 years along with all "brake like" (hydraulic fluid), i.e. clutch. It's cheap insurance to maintain the cylinders and lines free of rust, you'll be glad you did in the long run...
Why Change Brake Fluid?
So consider this: After three years of service, the average boiling point of the brake fluid has dropped to a potentially dangerous level because of moisture contamination and may not meet minimum federal requirements for brake fluid.
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View my fuel log 2016 Micra Active manual: 6.0 L/100 km ... 46.9 mpg (Imp) ... 16.6 km/L ... 39.0 mpg (US) ...