Replaced rear calipers, air in system?

[Pmj77151]

Hello all, I changed discs and pads recently and found that one of my rear calipers was all gooked up and seized. Two new calipers. They arrived today and I fitted them, bled the brakes and now I’ve got a brake warning screaming at me, and the pedal is sliding down to tbe floor gradually. With the warning I figured it was low fluid, but checked the reservoir and it’s literally topped. I’m guessing with the pedal moving to the floor it’s air somewhere, so will need to bleed all round, but not sure why the warning light is on when the fluid reservoir is brimming. What have I cocked up???