[firstgenaddict]

The issues are as BBBently stated moisture in the fluid, and lack of driving, the moisture pools and condenses then causes corrosion and then failure, second is that petroleum based fluids will eventually swell rubber brake lines, and the rubber seals/o-rings in the system.
Most of the original rear rubber lines (from body to rear end) I have removed have been so swollen they will not allow any fluid to pass.

If the system is currently not leaking and operational, I drain, replace any single crimp rubber lines and any old looking rubber replacements, then refill with silicone fluid. Since I am usually pulling and replacing the rubber lines I blow the old hard lines out with clean dry air, open the bleeders and blow all the old fluid out.
We have been using silicone fluid for 30 years and have never had any issues with brakes, including rarely used Corvette 4 piston calipers.