Soft brake pedal

[Nima]

Hello:
I am in the process of my LS swap conversion on my 1953 truck and I am having issue with soft brake pedal. Here is the history:
The 5.3 engine is from a 2001 silverado and the the brake booster and MC are form a 2001 suburban with the same size engine. I have bled the brake many times, the pedal firms up when the engine is off but as soon as the engine is turned on, the pedal gets very soft and goes to floor if pressed. YThe pedal has the feedback, it doesn't stay at the floor but you can just very easily push it to the floor with hand, no resisatnce? I thought the booster was an issue (junyard sourced for the mockup purposes) but when I changed that with the donor vehicle (which was running and stopping fine) I still have the same issue.
The MC was bench bled ( Ihave done these few times, not that I am an expert though) when installed, I don't have any leak from the lines that I can see. I have a mityvac bleeding system which I have been using.
Thanks for the help