The issue from the start seems to have been caused by a faulty DSC (ABS) unit. The DSC unit is used for cruise control, cornering brake control, stability control, hill start assist & ABS, possibly more. So it has the ability to brake individual wheels or axles itself when it needs to o so to fulfil any of the functions outlined above.

The last time I drove the car prior to the fix I had the rear brakes lock up solid. So I think the issue all along was that the DSC unit was constantly applying the rear brakes unnecessarily & causing the brake fluid to boil over time. The cause of the soft brake pedal.