""Soft" may also refer to there's still air in the system. Air might have got in during replacing those callipers. Repeat the bleeding process and see."

I would add to this a little. How did you bleed the brakes? Was it the "pump, pump, hold" method with a helper doing the brake pedal work? Was it a pressure or vac bleeder?

If it was the pump and hold method and you helper allowed the pedal to travel beyond its normal range, the master cylinder piston seals would have gotten eaten up by the "corrosion zone" of rust in the cylinder bore.

It the vac or pressure bleeder method, it has been said that the pedal needs to be pushed a couple times during the bleeding process to dislodge stubborn air bubbles...not sure I believe it.

""Swishing" sound may be due to leaking brake booster."

Or leaking check valve.