I work on cars/tractors most days so I deal with silicones a lot).
I clean the threads of the cap very well and then put some quality automotive RTV silicone on the threads (Dirko and Nissan brand RTV are my favorite. The white hard plastic o-ring under it is what leaks. On the back end of it is a disc shaped cap about the size of a nickel which unscrews with a large flat blade screwdriver. Anyway, the chamber that holds the CO2 just above the cartridge is the culprit. There's a couple small pieces and springs to keep track of, but it's not too difficult of a job if you take time and keep it all in order on the table.
Just joined this forum mainly because I have 2 15xt pistols (very accurate for cheapies, horrid trigger) and both developed the symptom of immediately leaking all the Co2 as soon as the cartridge was put in.
