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.