believe it or not O rings & seals have a shelf life,to about 3/4 years,so you dont need to service your PCP every year,only spring rifles need servicing once a year.
the trouble with O rings is it could be the slightest nik/split not visual to the human eye " well at least not mine ,unless I have me glasses on" and under pressure they will leak,so best to have a visual check, every few months for wear n tear,if you can use a magnified glass
regards
Davy