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The Sonnax (SON-28709) is an aftermarket heavy duty reverse piston return spring retainer. The SON-28709 is stamped from high-strength, roll-tempered steel, making it much stronger than OEM spring retainer that was manufactured from mild steel. If you are building a trans brake glide, most builders install higher-rate reverse piston return springs to achieve faster piston release. The stronger springs tend to deform the OEM spring retainer, the SON-28709 spring retainer was designed to work with the stronger return springs to help provide a faster, more consistent trans brake release.
