Pressurization issues

[Hwolf]

I bought a P337G 2 weeks ago, previous owner told me pressurization did now work well. Last weekend I flew her over the Sierras, and went to 12,500ft.
There was some semblance of pressurization, cabin was able to be 2000 Ft below the actual altitude, The altitude controller worked fine as at lower altitudes it would match the altitude setting, but was never able to get more than a 2000Ft difference.
I would expect a leak somewhere, be said the inflatable door seal is fairly new, but will not build pressure, pump works, makes pressure, but fails to stop, so there is a leak somewhere in that seal or the feed line for it.
Closing the door I can hear a clear hissing sound underneath the panel, so there has to be a valve there, where is that valve ?
I also looked in the Air scoops on the leading edge of each wing, both wings show 2 different flow routings internally. The maintenance manual I have does not specify where they go to, I do see there should be a Cabin Pressure check valve, where is that valve ? Looking into the right hand airscoop I see a brown/orange valve looking thing, its springy, is this the check valve ?
The left hand airscoop is missing that item.

I am very good in fixing things, done 4 STC's for Cessna, and one for Cirrus.
I want to fix this myself, as I really want to know how this airplane works.
She is a delight to fly, and I always have wanted to own a 337.

Suggestions ?
Thanks.

Henri Wolf.
Hwolf@airdata.US