Try softer rubber on the seal face.They sell this white "kinda foamy" sealing. Then maybe a slightly more powerfull spring that pushes the piston. And to prevent leaks, you may use epoxy glue instead of teflon tape on some threadings (which need not be dismantled for servicing or testing).

So I wanted something small to shoot paintball size projectiles and to be manualy powered. I used T piece brass piston valve (common design), shortened shrader valve for trigger, and smallest bike pump i could find for power source. The rest are brass fittings, teflon tape, epoxy glue and PP pipe fo...