TH 350 Odd Slippage

[Pampas56]

I just finished up replacing the transmission in my 84 C-10 and it runs fine and shifts great. The only issue is that when I first start it up it won't go into gear unless I rev the truck for about 5 seconds in gear. After that it is fine until I shut the motor off, then I have to go through the same routine all over.

I don't want to have to pull the tranny again if I can help it. It seems to me that the pump and converter both work fine and the internal parts are OK, but something is not letting me build pressure at idle.

Does anybody have an idea of what simple/stupid thing that I am missing?

Fluid and filter are new and fluid is at the full mark.