I got a similar dead batterie, it did rest on 11.30, one from the numbers what indicate me a dead batterie, but still had full amps.I did draw it down, and it didnt recharge anymore, only to about 4 V. At charging it shows 14 Volts, but later drops down back to 4 Volt. I used this thing a bit for my 3 Batterie setup. But i am carefully at this one, when its at 11.30. Usual each cell has about 2.2-2.4Volt Volts, x 6 Cells is about 14 Volt, when one has a short i come to 11.30 again. So it may means, that the other cells are full charged, just one is not and i should not charge it more.
I guess, that sometimes only the grid from that Plates fall apart and cause the short. I converted it to alum too but not much changes. You can try at least to discharge it with a low load, maybe about 5 watt, and recharge it few times again, that it can come back. Aaron did suggest that once, but i had more success at open it and refill it with destilled Water.