Still no brakes! Losing my mind.

I agree with the thought that you still have air in the system. When I was pressure bleeding out the completely dry system in my Dad's 69 GTX vert, it took forever to get all the air bled out. I thought I had the issue fixed and then the pedal was still low and would barely stop the car. I put the pressure bleeder on the car again and went through every bleeder starting with the furthest to the closest. I also filled a clear jar with some brake fluid and put a hose onto the bleeder with the other end into the fluid. While I would crack open the bleeder, I kept checking for bubbles coming out in the clear jar of brake fluid. Once the bubbles stopped, I let it go a little more and then closed it up and moved onto the next closest one to the master cylinder.

Your problem may still be with the Master cylinder. By plugging the ports on the master you had a firm pedal, but the minute you hook lines up the pedal fades. I believe the Bore in your Master cylinder is scored/damaged and once the little piston cups move inside the bore toward the damaged area then fluid just leaks past the cups and you have no brakes. I hope this makes sense, I think if you try a different Master Cylinder it will fix your headache.

Are the calipers for the vehicle or did you do an upgrade and change the calipers that are supposed to be on that application. Make sure that the calipers you pulled off have the same diameter pistons as the new ones if not you will never get a pedal.