Plinkster seal

I have one of these guns that I bought from MRODair. My gun came with a packet of seals. I have found that I have to use some sort of lever through the hole in the end cap to seal the gun when I insert a new cartridge. There is no seal under the end cap. I would guess there is a seal where the cartridge butts up against the piercing pin. Have you tried Tom Gaylord's cure of adding some Alphabet org fluid. This has worked for me on several guns.
 
The -009 O-ring will get you back in business. Somehow the seal on my CP1-M has never needed replacement, despite being both older and having gone through significantly more cartridges. Probably just another anecdotal example of how variable the quality control is. Fortunately they are super easy to work on, and they respond great to some tuning and TLC.

I just wish they would stop abusing the barrel crowns. First the Plinkster:


Fixed:


Then later, the CP2 for my son:


Fixed: