Or try jacking it up just a little more than you want, and then loosen the shackle and leafspring mount bolts. Jounce the car a few times, and then retighten. See if that helps.
Air has to leak from somewhere. It's possible the pressure raises when you drive (heats up), and only leaks then. Could be something cools while it's parked and it leaks.
Over tightening an oring can be as bad as a loose one. If you've tried tighter without luck, I'd try looser. Whatever the spec is for torque, try to hold to it or just under maybe (since the fittings aren't new).
Plastics are often not impervious either. There may be better hoses on the market, I don't know. If possible, I might try a direct fitting on the shock body with just a Schrader, no line at all. That would potentially eliminate more variables than anything else.
