Vehicles with a GM 4L60 (200-4R) transmission often have complaints of poor reverse boost or upshifts in manual low. This complaint is generally caused by a worn reverse boost valve sleeve or wear at the pump body. To resolve these problems, Sonnax offers larger ratio (.283"), factory-style reverse boost valve kit K77898A.
Due to the differences in the thermal expansion rate of steel and aluminum when the transmission approaches operating temperature, the sleeve has been allowed excessive clearance and scrubs the bore.
Contact a Sonnax Parts Distributor.