I think I found my problem. I took a look at the valve, and the valve seat seems to be missing an O-ring according to : part 28 http://cognitivevent.com/icd/images/tcat_schematic.gif
And it looks like the valve seat assembly has a delrin piece that is raised and uneven (perhaps a picture would be good to describe this)
Edit : It's a pretty bad leak. I only have 1k pressure on the 13CI, so I didn't really leave it on to see just how bad.