Persistent Lv1 lpr issues

I've had this Lv1 for about a year. When I first got it, I tore it completely apart and replaced all the orings and cleaned it up as it was filthy. It runs fine except for one nagging persistent problem. When the gun sits for a while, and I take it out and air it up the day prior to playing, the lpr leaks. I go through it to make sure the oring are good and not dry, and eventually replace the piston oring and it's good to go. It happens with the stock 013 oring and the suggested 012 90 duro oring. This repeats everytime it sits, and I do a pregame airup. It has since sat over the winter and today it's happened again today. I'm stumped...

I'd check/replace the small oring that rides on the piston stem.