Mostly covered by the knowledgeable gents above.
3 common place for leaks on the S4** series are..
Firing valve not seating properly
o ring under pressure guage.
The balloon thing is a good way to find out where the leak is. Place over the filler and see if it inflates. If not then place over the barrel (with breech closed) and see if it inflates. If it inflates on the barrel then it's a good bet that the fiirig valve is not sealing. If neither the barrel or filler inflates the balloon then it's the pressure guage and could just need 'nipping' tight. Normally replacing the o ring will stop the pressure guage leak.
you can get spares from Julian Bond or best fittings....or even Air Arms if money is no object
If you PM me your email address i have a s400 strip guide showing the rifle stripped to it's guts. It's an older s400 but virtually identical inside.