Vehicles with a GM TH400 transmission 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 inside diameter 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. Also, Reverse oil can exhaust both to the sump and back through the modulated TV orifice. The Sonnax boost valve kit 34910-05K eliminates the above problems.