Mystery band failures in GM Powerglides are a common result of using inconsistent aftermarket servo return springs and weak OE springs prone to collapse. This is a problem that can be traced directly to the pin bias effect. Sonnax race-calibrated servo return spring kit 28133-HD counteracts pin bias effect 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.
Watch this video to see exactly how an OE servo performs at high pressures when fitted with an OE, light aftermarket or Sonnax race-calibrated spring.

Counteracts pin bias effect up to 275 psi
Includes two .050” shims to aid in tuning at higher line pressures
Works with both OE and aftermarket servo pistons
Also available in Sonnax supported servo mater kit 28821-20K

More From This Category

Sonnax Oversized Two-Spool Relay/Signal Valve Kit, Aisin Aw, 09G/K/M, Tf-80/81Sc, Tf-60Sn, Each

Sonnax Oversized Two-Spool Relay/Signal Valve Kit, Aisin Aw, 09G/K/M, Tf-80/81Sc, Tf-60Sn, Each

$67.02

Sonnax Vacuum Test Plate Kit, Allison, Allison 1000/2000/2400 '00-'15, Each

Sonnax Vacuum Test Plate Kit, Allison, Allison 1000/2000/2400 '00-'15, Each

$845.00

Sonnax Heavy Duty Output Shaft, 4L60-E, 4L65-E, 4L70-E, Each

Sonnax Heavy Duty Output Shaft, 4L60-E, 4L65-E, 4L70-E, Each

$1,125.69

TCI 4L60E .490 Boost Valve w/ O-Ring.

TCI 4L60E .490 Boost Valve w/ O-Ring.

$121.27

Sonnax Reverse Servo Spacer Kit, GM 4L80-E, 4L85-E, Each

Sonnax Reverse Servo Spacer Kit, GM 4L80-E, 4L85-E, Each

$27.08