Since its introduction in 1969 with the 428 SCJ, the R ratio servo has remained the trick piece for C6 builders needing additional band apply force. Sonnax "R" ratio servo kit K36528R for C6 (mid 68-later) units features a no-leak pin which uses one O-ring and one PTFE ring to eliminate back pressure and leakage due to worn case bores. A replacement seal kit K36528R-SK also is available.
Some servo modifications involve blocking feed or exhaust passages in the case. These modifications must be removed. For proper operation of this servo, ensure that the exhaust passage and both oil feed passages are unrestricted.