June 22, 2023
Sonnax reverse servo spacer kit 34528-01K is a simple, low-cost way to add durability to the GM 4L80-E and 4L85-E transmissions and eliminates the need to retrofit early design reverse servo pistons into late units.
Due to a design change in 1996, the reverse servo piston commonly breaks because it is stressed more than early servos. Installation of this Sonnax kit reduces piston stress by increasing the contact area between the aluminum piston and spacer, and incorporating a cushion spring washer. The new contact area is over two times the original and the pressure between the piston and the spacer is reduced by 60%. This Sonnax kit easily installs during assembly and does not require machining or other modifications. Excessively high line pressure in Reverse is directly related to boost valve wear and cross-leaks into the EPC oil circuit. For maximum durability, inspect and replace the boost valve with Sonnax 4L80-E boost valve kit 4L80E-LB1, 34200-03K or 34200-10K.