Welcome to HPA PCP's ..... ANY of the primary O-rings, Poppet or it's o-ring seal, Fill foster, Dowdy washers or Gauge.... Sadly if you want it fixed your going to have to put forth some time in finding where it is leaking ?
Action out of stock and some spritzing of WD-40 around all the obvious joints, connections, fill fitting, gauge and dowdy washers. ** While liquid soap & water works far better, you don't want water and then rust in thos places that don't get taken apart & cleaned up. ** If your willing to commit in tearing the gun down and cleaning all the parts and pieces ? then soap & water will find you the leak a lot faster IMO.
Scott