'08 1500 Z71 - Brake help

[Tynee]

Evening gentlemen. I'm a new face to this particular section fot he truck board, but I've been around forever, so I'm lookign forward to getting to know the folks over here.

I just bought an '08 Silverado 1500 CCSB LT Z71 5.3 auto. It's a bit of a basket case right now. I'll be abck asking about engine codes in seoarate threads, but tonight my question is brake related.

1) My truck has RPO code JL4 - Active Brake Control, if I've done my research correctly. What is ABC, and what does it do for me?

2) More importantly, possibly related to 1), possibly not...I have zero brake pedal response. It doesn't build any pressure. I replaced the master cylinder (with a proper bench bleed and a power bleed at all 4 wheels) and took the rear drums off to make sure that they were properly adjusted. These 2 steps helped a LITTLE. Now I can build pressure by pumping it, but it goes away the next time I release the pedal. Am I having an ABS issue? No ABS light to indicate that. I'm a little lost here.

Any help would be appreciated.