Brake systems are not perfectly sealed
One important specification for brake fluid is the quantity of absorbed water it can retain while still meeting spec
Brake fluid is designed to absorb the water and allow it to mix so that it won’t collect in the wheel cylinders because that Would boil so fast and lead to dragging brakes which would generate heat to the point of the brake fluid boiling and then total loss of brakes because then the pedal is trying to squish a gas pocket and not solid fluid
There is a video on here posted a short while back of a fancy car fully resto mod and the owner is bragging about how powerful the engine is that he had to Ride the brakes to keep at the speed limit while cruising - totally dumb - and then he loses his brakes and crashed - as expected - because the fluid boiled in the wheels and pushed back the volume of fluid into the master

So when your brake fluid absorbed all that water
You need to replace it
With fresh stuff capable of absorbing new water
Or you die on the long down hill parts of the road
Or the flat parts if you have poor driving habits
-s