I had a Chevy Blazer that seemingly all of a sudden prematurely wore out and began metal to metal contact between the rear disc brake pads and the rotor. Naturally the rotors were ruined as there is very little metal that can be taken off them during turning  I replaced the rotors and brake pads and noted that I had a harder than normal time collapsing the caliper piston far enough to install the new brake pads.

These hard to collapse pistons and premature wearing out of the brake pads set off alarm bells inside my head, so I made sure to test drive the Blazer a short distance at slow speeds when we were finished with replacing the rotors and pads. When I started the test drive, I could feel the brakes holding back on the Blazer, and after only a couple of miles of driving the rotors were getting very hot.

I then disassembled the rear brakes and found that the pads were holding tight against the rotors with the brake pedal not being depressed, and had to be pryed apart using considerable force. I began the chore of diagnosing why the brakes were not releasing as they should, and deducted that the pistons not collapsing as they should had something to do with it.

I quickly eliminated the rubber brake lines having a flap inside them simply by thinking, both of the rear brakes were doing the same thing and also all the rear pads wore out prematurely. Having a flap inside a brake line would cause a problem with only one brake. The brake proportioning valve could also be eliminated because there was no problem with all four brakes holding as they should when stopping and there was apparently no problem with the front disc brakes dragging. In essence, the brake proportioning valve is a shuttle valve that comes into play when there is a problem with fluid delivery to either the front brakes or the rear brakes.

Upon thinking back to when I first started collapsing the caliper pistons in preparing to install the new brake pads, I remembered that the pistons collapsed easily at first then became hard before they were collapsed far enough so that the new pads could be installed. This more or less eliminated the entire system, including the master cylinder, because a problem with fluid returning to the master cylinder would likely make the collapsing of the pistons hard to do from the beginning, not just at the end of the collapsing.

All these deductions left the calipers as the culprits. Since the calpers were made of aluminum and brake fluid attracts water, impure water at that, the portion of the piston bore on the fluid side of the piston o-ring could possibly be pitted up and corroded from electrolysis (my deduction). Rebuilding the calipers would not be practical in a home repair situation because honing aluminum smooth is very different from honing cast steel or cast iron and, if the bore was pitted, probably would be impossible. If the caliper bore was pitted, then machining it back smooth and refitting it with an oversized piston would be necessary to rebuild the caliper.

I purchased two rebuilt rear calipers and installed them with no problem encountered in installing them along with the new brake pads and rotors. A short test drive revealed no brake dragging or overheating, so I made a longer test drive by returning the caliper cores to the parts store using the Blazer. Along the way I even tried hard braking when traffic would allow. There was absolutely no problems with dragging and overheating and the brakes worked very good.

Note:  The front disc brakes use a caliper made from cast iron or cast steel so the problems encountered with aluminum calipers would likely never happen or would be a very long time in happening.