69 ChevyC20 brake conversion problem

[Merbob]

I have recently converted my C20 to front disc brakes. I purchased the 8 lug conversion package from Brothers. I also replaced the master cylinder and prop valve with the ones they recommended. I bench bled the master cyl. installed everything according to their instructions, then bled all the wheel cyl. I have a strong pedal until I start the engine and the booster comes into play then the pedal goes to the floor and the brake light comes on. I called the CPP help desk and they told me to re-bench bleed the master, which I did re-installed and re-bled everything only to find the same issue. The tech dept. at CPP thought I might need a new booster which I didn't replace because it worked before I started this project. I have bleed the entire brake system no less than 4 times with the same results, anyone have any ideas?