OK so I have a 1995 Mercury Sable and when I.

OK so I have a 1995 Mercury Sable and when I purchased it, I thought it was fine. Then about 4 months later I heard the familiar sound of the squeal indicator from the brake pads. So that weekend I removed each tire and the brake pads were fine except for the right rear. This is a 4 wheel disc brake system. I purchased some new pads and just replaced the right rear. As I am driving I can feel the brake hanging up and within a month the pads were worn and had gouged the rotors. I assume the brake caliper was at fault so I purchased a rebuilt Caliper, brake line, a new rotor along and pads from Advanced auto and installed all items and also did the left rear pads. I thought I was all good. The next day I could feel the brake hanging. If I shut the car off for about 10 minutes it would free up sometimes It always freed up when the car sat for over an hour. Anyway the pads wore quickly and then I went to replace them and the cylinder on the new caliper would not turn into the caliper. I was using the proper tool which I decided to purchase after getting into this the third time. One I released the bleeder valve the cylinder began spinning into the caliper with ease. I then tightened the bleeder and again it would not turn in. So I am thinking something else in the system, is not allowing the fluid to return. So I trace all the lines and to me it appears after the master cylinder there is a square metal piece right along the left front tire inside the engine compartment, them from there it appears that they head forward to a pump which I believe is the ABS pump located under the battery. I guess my question is what would be the most likely culprit at this point? Master cylinder, ABS pump, or that thing mounted to the frame? At this point I am thinking of heading to the U pull it and just grabbing all three and replacing one at a time. I want to give this car to my son who just got his learners permit but I will not pass it along until this brake issue is resolved.