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  • Broken Low/Reverse servo piston
  • No Reverse


Poor reverse servo design and high pressures in reverse combine to cause reverse servo breakage.


Install the Sonnax kit to increase piston to spacer contact area, reduce stress on the underside of the piston and add a Belleville spring washer between piston and apply pin.

4L80-E, 4L85-E

Reverse Servo Spacer Kit
Part No. 34528-01K

View Instructions

The Sonnax Reverse Servo Spacer Kit 34528-01K is a simple, low cost way to add durability to the 4L80-E transmission and eliminates the need to retrofit early design reverse servo pistons into late units. Due to a design change in 1996, the reverse servo 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 34200-03K or 34200-10K.

  • Increased contact area reduces pressure between apply pin and piston by 60%.
  • Added Belleville spring washer reduces shock on piston.
  • No machining necessary, drop-in components.
  • Maximum durability for minimal cost.
  • Short Spacer
  • Belleville Spring Washer
  • Flat Washer
  • E-clip
  • 34528 01k combo
  • 34528 01k
  • 34528 01k alt
  • 34528 01k details