Th400 shift problem

[Whamp48]

Th400 won't shift out of first gear. It will go out of first, but doesn't deem to be in a gear , then when you slow down, it goes back in first. Did all the suggested fixes, modulator, checked valve body, cha ged fluid, added trans-x, still same thing. First, reverse, neutral, and park. Today I took the valve body off, and while inspecting it, I pushed down on the servo piston with my hand and fluid squirted from the kickdown sensor. I guess that's what it is. It looks like a gauge sensor. It screws into the valve body by the servo piston. I can't find one listed by the parts people. Can I just plug it and let my kickdown be done manually? Thanks for any knowledgeable answers.......Warren