Brake Bleeding air volumes?

[2005GMC1500WT]

I have a 2005 GMC Sierra WT 2wd, Standard Cab with a 4.8L. Just replaces all the brake lines as the old ones were rusted through. Bench bled the master cylinder and now trying to bleed out the brake lines. I've already pulled about half a large bottle of brake fluid plus LOTS of air. I realized it will take a lot to refill the lines but it has been pulling mostly air. Using a vacuum system with the air compressor. Started with the rear passenger side then go to the rear drivers side then front passenger and end with the front drivers side.

 

Feeling a bit perplexed. How much air and fluid should I expect to bleed out before the lines start filling? Is it possible there's an air leak in the brake system? Such as a line not properly attached?

 

Thank you.

[Chubbs878]

Did you bench bleed the master first?