How was this confirmed?
Car in the air, wheels off the ground, someone pressing the pedal, while the rear tires are checked for brake application??
You could also flip up the caliper and make sure it moves when the pedal is pressed, just be careful to make sure that multiple pedal presss don't push it out too far. If you wedge something in between that won't damage surfaces, it will keep the piston from popping out.
Next use large channel lock pliers or a brake piston screw kit to push the piston back in .... oh, these cars probably have the "twist in/push in at the same time" rear calipers, so try using a set of large needle noise pliers and inward pressure (your hands are going to hate you and don't get stabby with your hands in case you slip) and make sure the caliper pistons will 1: Turn and 2: Retract.
Once caliper bore/piston movement is confirmed on both sides, then go to the next step.
Have someone operate the parking brake. Be sure to confirm the pistons move out, as they are acted upon with direct physical/mechanical operation of a pivot/arm inside the caliper. If during these tests you don't get movement from physical actuation of the piston directly, then try this.
Go with a bleed, but be very weary of old bleeder screws. Snap one and you are either attempting to drill/easy out the screw or putting on new calipers.
I've torched a myriad of screws, let them cool just enough from being cheery red, to then allow light pressure manipulation to crack them loose. Sometimes they don't play nice and require a few applications, and other times they require no heat at all.
Wrote a book, but just wanted to make sure the basics were covered.