Conventional GM Powerglide servos are not well-supported and the return springs do not function properly, both leading causes of band failure. Sonnax supported servo master kit 28821-20K is an all-in-one kit that eliminates piston binding and includes a race-calibrated spring to counteract the pin bias effect.
At high apply pressures or with a worn servo pin bore, the servo piston is side-loaded in the case, causing friction and binding of the piston. The supported pin design of this Sonnax kit eliminates friction caused by side-loading and ensures the servo piston will apply and release without binding.
"Mystery" band failures are a common result of aftermarket servo return springs and weak OE springs prone to collapse that do not reliably release the servo. The Sonnax spring in this kit is race-calibrated to ensure consistent servo function for longer band life and a cooler transmission. The spring's higher seat pressure helps ensure full servo release, but also features a lower spring rate to prevent excessive release force in the apply position.