Spongy brake pedal

First off, I know my way around vehicles but, I need a second opinion.
I have since new vehicle listed above sle z71.
It's been retired as a chore truck.

I just put on all brake system components except master cylinder.
Literally, front bearings, front abs sensors, calipers, flex hoses, rear wheel bearings, seals, wheel cylinders, drums, brake shoes, pads, flushed with new fluids til clean and clear.

I have a motive pressure bleeder and I purchased an autel maxicheck to actuate the abs motor but the maxicheck goes back today because it will not auto bleed this year model.

I put the truck on jack stands to perform all the repairs.
While on the stands I bled the system and placed the vehicle in 4wd, started engine, put in gear and reached a speed of 20 mph, applied the brakes moderately and the abs kicked in for approximately 45 seconds. I did this several times then bled at wheels again and no air was seen exiting the bleeders.
The rear brakes are slightly touching the drums and all hardware is cleaned lubed and working as needed.

I still have a spongy pedal! Will not get firm until past 3/4 way down.
I plugged both outlets to the brake lines at the master cylinder with brass plugs. The pedal is firm and has little movement while these plugs are in.

I am okay with taking it to someone with tech scanner but, is it possible my master cylinder is the issue?

When I start the engine is when the pedal goes to the floor.
No leaks anywhere and everything was original. I know the rear circuit went dry when the wheel cylinder piston seal blew.
What am I missing here?


What do you think? Air in abs or do I have a master cylinder issue?

Thank you