Your rifle is likely leaking at the valve. The valve stem has an oring made into it. If it's the valve oring, you'll need to purchase a new valve to correct the leak. It's best to just buy a seal kit with all of the orings / valve stem and replace all while you have the rifle disassembled.
Send me an email if you need further assistance or advice on the repair. Usfieldtarget@gmail.com