Is the hiss from inside the breech or at the seam where the can and breech meet? Either place, it is most likely just a worn o-ring. If it is inside the breech, check/replace the o-ring right behind the feedneck (size 017 70 Shora A) and the o-rings on the bolt stop (014 70 shore A inside o-ring 020 70 Shore A outside). At the breech seam - there are 2 o-rings on the breech that mate with the inside of the firing can. Both of those are significantly less likely to go bad, and it is very unlikely that there would be a leak there without being able to notice it down the barrel too as it would mean at least 1 interior o-ring was bad as well.
It could theoretically be the sail o-ring on the bolt as well - just a bit less likely given that it is new. That is a metric o-ring though, 20mm OD x 16mm ID.
Glad you like it, let us know what ends up being the problem behind the leak.