Mushy brakes 4 life...

Disk bleed nipples on top to get air out?
Ok.
I'm thinking the rear symptoms may be masking a simple problem.
I would start a square one.
Bench bleed the master cylinder.
Put a finger over the outlet and push the piston.
See if it squirts air/fluid past your finger under pressure.
Keep the finger on the outlet as the piston returns and it will suck the fluid from the reservoir to where it need to go in the master cylinder.
Put a plug in the hole you just did loosely pump/tighten it to preserve the "prime"
Do the same on front and rear.
Don't pour anything out.
Put the thing back on the car.
You won't loose much it you do it right away.
And then have someone help you bleed it again at the threaded connection like you would nipples.
Loosen, pump, hold ,tighten.