Replaced prefilter twice, orings once, lines with new orings once. At this point it could be a nick in the prefilter housing (not likely), but the vacuum test negative pressure holds for the housing and lines from the heater to the SOV. So it's a test from the prefilter to the tank this weekend as well. Could just be a dirty tank screen. Who knows?

I have a quick question though I could probably easily answer it myself. What kind of "seat" is behind the SOV (other than the oring). The "nipple" on the back of the SOV was kind of hard to get to go back into it's "seat" when I reinstalled it this time 'round.