Brake fluid is hygroscopic, and it doesn't have to absorb a lot of water to lower the boiling point of the fluid to a dangerous level.
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In addition, the moisture can cause corrosion in the system. All of my cars get the DOT 4 brake fluid changed every two years, with the exception of the track rats, which have the fluid changed no longer than six months before an HPDE.
Yes I'm well aware of how brake fluid functions and how changes affect the performance, but a fluid exchange every 15K according to Nissan IS total B.S. and I will call anybody out on that. People should also take into account of location/uses a vehicle is being driven as environmental factors surely play a large part.