I wouldn't advise re-using any seal they cost pence.
Have you bled the master cylinder? I changed the line on my bike earlier this week and had to bleed the MC via the banjo bolts as I was getting nowhere using bleed nipple on it once I got the juices flowing I bled the calipers then back up to the MC bleed nipple. I did tie the lever but there was no need. Another trick you might use is to unbolt the calipersand wedge the pads then bleed them holding them above the MC, there might be some air trapped in them.
If none of these work the strip them and replace the seals. Apart fron creating a seal the pistons rely on the elasticity of the seals to return them into place, actually I'd replace the seals anyway but then I'd never take a chance with my brakes.
I've tried all sorts of gizmos for bleeding my brakes and the best I've found is a length of pipe, bottle half full of fluid, care and patience.