If the rear brake pistons are rotated 45° from the way they should be, they won’t lock into the self adjusters.
If the self adjusters aren’t engaged - you won’t have enough throw in the cable system to lock the brakes securely.
So that could be the problem. There is a groove on the face of the piston, that should be oriented vertically. This will allow the self adjusting function to ratchet. There should be a little tab on the back of the brake pad that fits into the slot on the piston, and actually holds the piston in the correct orientation.
You will have to remove the rear brake pads to see if the piston is in the right orientation. Or if the caliper is off of the car, Just slide the brake pad forward and you should be able to see what I’m talking about.
You can rotate it with a big screwdriver.
I know what I’m saying is a little cryptic if you don’t have a brake caliper in front of you. Once you see the face of the piston, it should be obvious what I’m talking about.
Rocky