hi, first is your filling gauge reading/working as it should, if ok then the hammer spring broke/weak or hammer seizing up/stiff, all that can be looked at without emptying the res as its on the non pressure side of things, if ok then its go to be emptied of air. quite possible the valve stem at fault bent/stiff/deformed . if you feel confident and know what you are doing then that's the way I would go. if any doubts then get it looked at. hpa is a dangerous thing. atb gc...