Update!
I think I fixed the leak
Opened the valve and found no "damage" (weird?) on any part. Closed it, connected the CO2 and put it in a bowl of water to try to pinpoint the leak. It was (as I thought) leaking from the front. Took the valve apart again, and added a washer "behind" the spring (to give it more power, in attempt to effectively hold the valve closed tighter). Connected to CO2 again, water tested again, and there was another leak, this time from the bottom of the valve (LOL). This one was quickly fixed with Teflon tape.
It doesn't leak anymore, but the sound when shooting is a bit different (or maybe I'm hearing things.)
Anyway, I'm pretty proud of myself (first airsoft repair
haha)