03 mustang brake troubleshooting

So dumb people that weren’t me failed to maintain their car and ended up running all the way through their rear passenger brake pads and it spit out the inside pad at somebody point in time. It ended up spitting the piston out the caliper too so that’s fun. It’s then driven for an indeterminate amount of time after and then handed off to me.
I Replaced said caliper and can’t get fluid to come down the line. Front brakes are bled and work (sorta). Peddle sometimes has pressure at 75% down, other times it dumps straight to the floor. If I bleed the fronts it’ll draw fluid out of them resivoir, but not if I bleed from the backs.

My guess is that air got into the ABS modual or even further upstream. Anyone else know or have similar problems?
 
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I remember something about different years(with ABS)could be bled normally but newer ones needed a controller for the ABS Pump to get all the air out. I think it was OK up to '98. When I get a chance I will try to nail it down.