If you leaking from the pressure line there are couple things I would look for.
I would pull the line off the PS Box and inspect the brass cone in the box. Make sure there is not any debris in there that would keep it from seating and that the cone is not deformed from previous use . And make sure you have the correct flare type. You didn't say where the box came from.
Then I would question how you are tightening it down. I treat them like brake lines. I will tighten it up, back it off, tighten it up again and back it off a few times. This helps seat the flare and you can usually get a half a turn more out of it. I also use a line wrench style crows foot with a long extension to get in there. Not sure how I ever lived without. You can get a whole lot more leverage on that nut.
