I have a braking problem and need help !

I recently purchased a 2002 Aprilia Tuono in a private sale. After the ride home and a few subsequent rides the brakes felt a little 'grabby' although still had good stopping power. An inspection of the Brembo calipers (non radial) revealed that at least one of the pistons in each caliper appeared to be seized. being fairly confident with bike maintenance I set about stripping the brakes, removing the pistons and all seals. I then cleaned up the pistons and calipers. I reassembled reusing the original seals as they seemed okay.

I thoroughly bled the system using a one way bleed valve and also left the lever tied to the bar overnight but the brakes still feel spongy. I've bled the system several times and there doesn't seem to be any air in the system, the brakes aren't awful but lack the 'bite' I'd expect or had before.

Does anyone have any tips/ideas. Do you think it might be because I didn't replace the seals ?

Any help would be much appreciated.