Brakes soft after sitting a few years

[Dead Parrot]

Yes, internal unseen leakage. Had something similar happen shortly after I bought a boat. While towing the boat, the brakes suddenly went soft after a couple of hard stops on the highway due to merging traffic. First trailer that truck had towed in over a decade. The hard stops pushed the seal into previously unused territory and it found rust. Replaced the MC and all was good thereafter.

If your MC is a step or two up from the normal plain parts store MC, might see if there is a rebuild kit. Probably a pass or two with a hone and a new set of seals will fix it.