• Low line rise
  • 1-2 Soft
  • 2-3 Soft
  • Delayed Reverse
  • Reverse shudder


Reciprocating boost valve wears the inside diameter of the boost sleeve. A worn boost sleeve.


The Sonnax drop-in boost valve kit restores hydraulic integrity and provides a positive seal with the pump bore.

TH400, TH425

Boost Valve Kit
Part No. 34910-01K

O-Ring Style

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Vehicles with a GM TH400 and TH425 transmissions frequently have poor line rise, soft 1-2 and/or 2-3 shifts and/or Reverse engagement difficulties. These problems can be caused by the reciprocating boost valve wearing the I.D. of the boost sleeve. When this occurs, oil that enters the modulated throttle valve orifice leaks past the boost valve and exhausts through the Reverse orifice or to the sump, resulting in poor line rise. Reverse oil also can exhaust both to the sump and back through the modulated TV orifice. The O.D. of the boost sleeve also allows some leakage through the pump body, which can increase boost oil loss. Sonnax boost valve kit 34910-01K eliminates the above problems by restoring hydraulic integrity and providing a positive seal with the pump bore.

  • Valve and sleeve manufactured from high-quality steel
  • O-Rings provide additional protection against leakage through the pump body to assist in maintaining pressure in the circuits which supply the boost valve
  • Uses the same valve spool diameters as the OE version to retain factory shifts
  • Drop-in Zip Valve parts install quickly and easily
Learn More About Zip Valve™ Drop-In Repairs
  • Valve
  • Sleeve
  • O-Rings (2)
  • TH400, TH425 Boost Valve Kit 34910-01K
  • TH400, TH425 Boost Valve Kit 34910-01K