Worn reverse boost valve sleeve or wear at pump body.
This large reverse boost valve assembly is made from hardened steel and O-rings have been added to seal the reverse boost circuit and prevent leakage.
Vehicles with a 4L60 transmission often have problems with poor reverse boost or upshifts in manual low. These are generally caused by a worn reverse boost valve sleeve or wear at the pump body. Sonnax offers a larger ratio (.283") reverse boost valve kit K77898 to restore the hydraulic integrity of the assembly.
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.