Many outlets for o rings around. harbor freight sells a kit with many sizes for under $10.
Mine wasn't releasing either, so when I depressurized the hose, it lost a couple hundred PSI in the Marauder tank. Crosman advised to put some silicone oil on the tip of the fill valve each time I filled it and see if it would fix the problem. Did it for about three fills and the valve started working like it should.
If you try that, make sure it's silicone oil, as normal oil will combust at the pressures a PCP works at