I am using an old leisure battery to power some security bits in the garden but it was going a bit flat. Just looked and its hissing and emitting vapour while on charge. Why? Didn't get too close with a fag in hand!
Sorry, I dont know either but I had the same problem last month. Bought a caravan to use at trade shows and the battery did not hold a charge, after charging it, it had the same symptoms a yours but mine also started to leak what I imagine to be acid, this lasted for a couple of days, so left it well alone till it stopped, now it is at the tip!
If its ever been allowed to go flat or even below 50% full charge its propably scrap. Lead Acid batteries will last for years but only if they are kept up to charge or dry stored. You can get get "rescue" kits i.e. little tablets you drop into the cells but they are a waste of time. Recycle and get a new one.