Psi rating for radient floor heat panel test???

Well gents, you guys have helped me with everything from thermostat install to asphalt driveways, so here is today's conundrum.

As many of you know, I built a pole building with radiant in floor pex heating. I pressure checked the " in floor" portion and it held 50 psi for months and months.

Fast forward to today...I built and soldered the boiler/wall panel. Figured I would bench test it also. On saw horses in my basement I'm embarrassed to say, I have struggled with pressure testing. Surprisingly to me, my solders are perfect. My NPT fittings have however nearly ALL leaked with soapy water. Some ptfe thread paste, some Teflon tape, doesn't seem to matter. Tightened to fear of cracking.... can't seem to hold more than 35 psi in 24 hrs. I have repaired/redone all leaks used soapy water. Still leaks down from 40 to 30 over night. Seems to really hold there continuously.

I'm contemplating throwing in some leak sealer on startup, and letting it rip (they do sell a purpose built (base hit 2) sealer for new installs.

Operating pressure on an in floor system is 15 psi.

Thoughts??

Thanks for listening to me ramble and any assistance!!!

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