the dump valve guns are best stored empty from a piece I read as generally the charge sneaks past the quad seal-not 100% sure on this myself but I tend to cycle my 130's/1300's - 140's and my 760 once every month same as I do my firebird and spitfire
I have a bam b45-3 and have left that charged for long periods along with sharp innova's and ace's but they use a moving sealing pin unlike the crosman's that uses a sealing cap