Brake issue and I am at a total loss.

How are you bleeding the system - two friends are better than one or none - and lots of brake fluid and tight shoes. It can be a nightmare. I think the lines are to blame - too many high spots for air to accumulate and not enough flow. Get friends - no radio or TV noise - make them pay attention -know what they are doing - and pump the **** out of each line - bleed the master lines at the master, bleed at the PP valve if you can get them loose, bleed front to back then back to front - test pressure again and again. Been driving mushy brakes for years - they aren't all that great to start with and not enough friends or kids willing to help bleed brakes. Promise of Beer or Treats sometimes helps. Tried all gimmiks and systems - Been there done that.
Still doing it.
Good luck.
Bcudamatt