Mr Lee, I've found flat retainer rings installed both ways on OEM (read, incredibly scabby) rear calipers. in addition, all the rebuild kits now use round wire retainer rings everywhere. On some caliper bodies, the smaller flat ring or the round rings tend to pop loose before they take a set, so if you're having that problem, use the wider ring at that point (after wire brushing it clean). IMHO, any boot retainer that holds on tight is better than the alternative.
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