Servo Piston Return Spring

Part No.


Servo Piston Return Spring
  • Servo Piston Return Spring



Servo Piston Return Spring

Part No. 28133

The servo return spring is the source of many "mystery" band failures caused by inconsistent servo release and weak springs prone to collapse. Sonnax servo return spring 28133 for GM Powerglide units is a lighter-than-stock spring ideal for low line pressure applications of 175 psi or less.


If this spring is installed in applications with more than 175 psi, the servo may not fully release due to the pin bias effect, resulting in dragging and burning of the band. Sonnax 28133-HD is recommended for higher pressure applications.

Related Parts


Prevent "Mystery" Powerglide Band Failures by Overcoming the Servo Pin Bias Effect

Unexplained band failure in Powerglides is a common problem that can be overcome by understanding the servo pin bias effect. This video demonstrates how the servo performs at high pressures when fitted with an OE, light aftermarket or Sonnax race-calibrated spring, available as a standalone component or in Sonnax 28821-20K. The video dramatically shows the pin bias effect in action, and why overcoming this built-in design flaw of the Powerglide servo is the only way to prevent “mystery” band failures.

Learn more about the servo pin bias effect.

Contact a Sonnax Distributor to purchase this part.