I've refilled the tank, but problem is still there. I've also replaced the large valve nut o-ring, and got another o-ring for the powertube nut. Leak is still there, which leaves only the cup seal as the main possibility. Its strange though, as the cup seal looks like a new one. Maybe I'll try lightly sanding and then using polishing compound where the cup seal seats, in case there is any tiny imperfections there I can't see.
I'm also curious about whether anyone has used one of these on a trracer with any success LAPCO Nelson/ Phantom CNC Machined Cup Seal | VIPs Save 10% & Free Shipping - Paintball-Online.com