April 27, 2016
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 piston 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.