Don, here is my experience which sounds similar to yours. I bought the neat stainless tubing, the AN3 fittings and one of the hydraulic forming tools that makes the flare.

Set out building brake lines for a 4 wheel disc setup. Looked like a million bucks but leaked like a Democrat Senator keeping a secret. About half of the fittings leaked. Tried multiple fittings..... with no improvement.

Used a magnifier to look at each joint and found that the problem was the stainless. Andy and others are right....NO teflon tape or sealer, the interference of the cone on the fitting makes the seal. A 45 degree fitting actually has one side cut at a slightly different angle.....just like a engine valve seat. The seal is made by a narrow contact circle just like the engine valve. If you look at your tee or other block where the tube contacts, you might see a depressed ring....chances are that these seats will leak next time that you try to install a tube.

My solution? I went an bought lengths of regular steel tubine and tossed the stainless. Every leak stopped. Doesnt look as cool but you can polish it and then apply a clear coat.

mike in tucson