Just found this. Maybe your fluid had some water in it?
“Another problem with moisture in brake fluid is that it causes corrosion. Moisture laden brake fluid begins to attack the metal in the brake system. Experts disagree on precisely how long this takes. Most agree between 2 and 3.5% moisture per year may be absorbed.
Much of the modern brake system is comprised of aluminum, a highly reactive metal. Oxygen in the absorbed moisture combines with aluminum to form aluminum oxide. This damages the metal components. Once the surface finish of hydraulic cylinders are corroded, they begin to leak and must be replaced”
Obviously this is refering to a car system but basics and materials are same.
Have you ever bled the system with old brake fluid…over time it absorbs moisture from the atmosphere.