Chevrolet Repair: TH350 trans, intake leaks, ok check

Question
I have a TH350 that was rebuilt (twice now) and still won't shift out of first. If you have it in drive, it slips. If it's in manual L1, it's solid but still won't upshift manually. The kickdown cable has been disconnected and doesn't change anything. The modulator is connected to the back of the intake. I'm a chevy tech, but don't do tranny's. Our guy said he didn't find anything when he tore it down. Ideas?

Answer
Hi Bret... Check the vacum to the trans...make sure you have lots...(no intake leaks) if thats ok check kick down cable make sure it travels all the way in and back, hook it back up and set it by holding the button down and advancing the throttle all the way. if none of that works then check trans fluid pressure make sure it is within  spec and full of fluid...sometimes those trans need to be a litre overfull to work right...if all that ok then problem is inside...maybe bad case leaking pressure past seals....hard to say... usually those trans are bullit proof....hope this helps