2002 GMC Sierra Brakes spongy

I have a 2002 Gmc sierra SLT that I just picked up a few months ago. The issue is that the brakes are awful, and they appear to be an issue the previous owners were chasing down too, all calipers, rubber lines, and hard lines have been replaced.

While they do stop, they feel extremely soft and spongy, and go close to, but not at, the floor for just normal braking. I have bled over 1.5 gallons of brake fluid through this thing, with the master cylinder level, raised, and angled down, bled manually and vacuum bled, and I have bled the ABS pump. Any ideas as to what the issue might be?

Edit: Brakes are Vacuum assist
also, something that throws me for a loop is that they don’t fade