Constant leaking co2 from paintball gun?

This once happed to me. The valve on the CO2 canister could be messed up or you could have a leak/crack in your hose. In my case my hose was messed up. what i did was wrapped the hose in tape to see if this was the problem which it was. so i brought it to a shop and got a new hose and the problem was solved.

Did you lube the O-rings before installing them If so check your cup seal it is hard to tell you what is wrong without knowing what model the marker is but I would have to go with the cup seal assuming you replace everything correctly

Without knowing the gun type its harder to give specifc ideas See if you can feel/hear where the leak is originating to help narrow it down. Check the ASA, if the threads are furred up it can cause it to misalign. Re-check that the o-rings are all ok, the right size/type and seated properly, also check for any muck or gunk anywhere, even a small amount can be enough to create a leak. Did you lube it all up correctly? Even though its leaking does it recock when fired? If it doesnt check the bolt o-rings and valve seats. Also check the micro hoses havent cracked. How long since it was last used/cleaned?

might be as simple as a scratch on one seal. if the Oring doesn't seat properly then it's useless.