Often, I've had to tighten and loosen new fittings firmly several times to get a good seal on new lines. You should notice the threaded nut turning in more-and-more as the flare finally seats.

I also tap the master cylinder with a small hammer to help expel the smaller air bubbles while on the bench.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)