HW/Weihrauch The hw44 - how’s it aged?

I bought a used HW44 in 177 a couple months ago and now it's blowing air out around the magazine. Anyone had this issue with their 44? What was the fix? I assume it's the breech o-ring!! Is that part #28?? Any experience replacing this o-ring and exactly what o-ring it is? Any other thoughts other than the o-ring. I haven't attempted any fixes yet.
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Not sure if it's #28 or not. Haven't been able to determine what the #28 o-ring is exactly. Don't know if the barrel needs to be removed or if there is access thru the magazine slot.
Any thoughts out there??