The Master Cylinder has two chambers. One applies pressure to the RF/LR The other applies pressure to the LF/RR The ABS module does manage both of these circuits (Which is why it has 2 inputs from the master cylinder and 4 outputs one to each wheel) If the piston in the master cylinder doesn't retra...

Thanks for the photos, they are very educational! That is precisely the reason brake caliper seizes: the cover boot is cracked (could be road debris or old age etc.), allowing water/salt to get in corroding the piston. Once the piston is corroded, it won't move any longer...or it moves but does not ...