There are only 4 seals in the entire gun. Cup seal, valve cap o-ring, a small o-ring behind a flat headed screw on the lower end of the valve assembly (plugs the end of an air passage way that had to be drilled after casting the assembly, hardly ever goes bad), and the 12 gram puncture pin.
That's it. Check those 4, replace as necessary. Though a leak through the cup seal can also be caused by a broken powertube (if broken a perfectly fine cup seal might not align properly).
David Johnson, AKA Fubarius.
Nunquam emere potes facere