Chances are that the DOT4, combined with the lack of driving, is what's killing MCs. DOT4 is extremely hygroscopic, much more so that DOT3, and that water causes corrosion in the bore. When the seals ride over this corrosion, they get damaged, and cause leakage.
If you don't rely on this car as a DD, then I would pull the master that's on it, and rebuild it myself. Pull it, disassemble, and check the bore the whole length with a T gauge. If it's within specs after being honed, then get a kit for it, reassemble, and be done with it. If you still use DOT4, make sure to get a compatible seal kit. Replace your grommets and fill cap seal at this time as well.