Lonnie,
Since you are set-up with a test tank why don't you test it like a radiator? Take and block one hose put a fitting on the other for a quick connect and use a compressor. I agree with your figures on pressure just adjust it to 5lbs and watch it close. If you don't see that core bubble anywhere after say 2 minutes it is the hoses. If there is a bad spot on the core 5 or even 4 lbs of pressure will show. You can also try testing it in a couple different orientations, you never know?
Good luck! I have been following this with interest.
