I've been through this myself about 87 times. On the Tahoe what finally made the brake pedal firm was about 3 panic stops form 35ish MPH. On the project truck, we just bled and bled and bled. It was finally ok after weeks of that until the axle swap and then back to square one again. I don't think ABS works on that truck because of the new PCM. I'm probably going to ditch the ABS to be honest.