The front brakes of a car do most of the work in stopping a car, so they are usually larger diameter, with bigger brake pads, and the rotors are vented because of how hot they get during use. The rear rotors on most passenger cars are not vented, because they don’t have to do as much work and get as hot.
When the technician working on this car took off the front wheels to check the brakes, he found something surprising. Whoever had replaced the front brakes had used rear rotors on the front. It was obvious because they were non-vented rotors. None of us had ever seen anybody do that, and didn’t even think it was possible, because there’s no reason to ever try.
Because the rotors are non-vented, they are also much thinner. As a result, the caliper piston had to move out a lot farther to engage the pads against the rotor. If this car had been driven long enough that the pads wore down, it’s possible that the piston would have to push out too far, and maybe begin to leak or even come out.
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