Have you serviced the brakes on the vehicle recently as some vehicles require a scan tool to bleed the air in the hydraulic brake lines if you have ABS.
I would first inspect for fluid leaks seeing as you are having a softer brake pedal.
Did you look at the rear caliper guide pins to make sure there lubed correctly and not frozen. Did you try to retract the rear pistons in the calipers to make sure they are not forzen also?
I think a good visual inspection is on order starting from the rear of the vehicle.