So, If I understand you correctly, one of the cells is draining faster than the others,
so then I assume that that one cell is faulty (and hence the battery can also be deemed as faulty)?
Yes that sums it up nicely, according to your info it has 7ah of capacity remaining.
As it is a wet battery you could check the fluid level of the individual cells, warning it contains battery
acid and you don't want it anywhere else but in your battery!
If fluid levels are ok (if the battery is white and a bit see-thru you can see from the side, it should be between min and max, if not visible
from the side you can look on the top by unscrewing the filler caps -battery must be on a flat and stable surface-
and shine in the holes with a torch, the metallic plates must be completely submerged in
acid .)
If levels are low they could be topped of with distilled water.
When fluid levels are ok charge it back up to 14,4V and discharge again to the moment the start of the drop off becomes visible.
Repeat charging/discharging to see if it improves.
Odds are slim but could be worth a try..
Edit: what Seanb says