We are pretty sure it just needs a new master cylinder and rear brakes adjusted.
When you hit the pedal it pops then brakes go to floor.
So thinkin the piston in the master cylinder sat in one spot for so many years that surface rust happened in rest of cylinder and that when it was bench bleed it messed with the piston seal causing damage to it and allowing fluid to go by.
As it is right now rear brakes don't engage at all when brakes are hit and the fronts immediately lock up once you hear and feel a pop as you are hitting the brakes.
Just need to order a master cylinder