I just took the spacing orings off my probe and when I put it into the hole up to the shoulder One of the Sealing orings was visible on the other side of the cylinder. If I tried to fill the rifle air just rushed out just as you described. If I pulled the probe out by the thickness of the two spacing orings the probe sealed itself and no air leaked out.
This must be the source of your leak
FWB P8X,Hammerli AP40, Steyr LP1 Walther LPM-1, CPM-1, CP1, CP2, LP3, LP53, LP300, LP400, Terrus, Pardini P10, FX Wildcat .177, HW100 .22, AA S410 .22, BSA R10 MK2 .177, , HW77, 80, 90 BB AK47, S&W 586 and more blow back Co2 BBs than you can shake a stick at
Is it the slow fill type with the air restrictor? If so try and find someone who has a fast fill type. The slow fill type can cause all sorts of problems that only a quick blast of high pressure air can fettle. I drilled out the hole on mine to remove the restriction as I couldn't fill my S400 when it vented. Never had a problem since.