If you didn'ypush the pistons as you turned them in, it's possible that you ruined the rear calipers.
But that would not account for a soft pedal.
I suspect that you didn't change the brake fluid every ofteh (supposed to every two years), and gunk build up was released into the master cylinder (which is now preventing the seals inside from working) when you retracted the rear caliper pistons.
In some cases you can salvage the brake master cylinder by bleeding a large voume of fresh brake fluid through the master cylinder in an attempt to flush out the debris.
If that doesn't work, you are going to have to replace the master cylinder.