mine started leaking a couple of weeks ago, although it was a lot faster. it was the fill valve in the end(£10 posted fron bar) spit on your finger and hold said spit over filling valve, look for bubles. if they are very slow, a quick blast of air might fix it. also my cylinder was loose in gun although i dont think it was leaking there. never left my gun unused for that long but i suspect there will always be a slight loss if you are talking months.
hope this helps