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.

[Pontiac1976]

You say fluid it's at the full mark is the after it warm up and running?
Because it sound like low fluid. or filter maybe out of place.
If the two things are ok then something in the trans sticking.

Oh by chance do you have a trans cool on it. If you do make sure there no air traped in the lines.

[cadillac_al]

My K20 plow truck has been doing this for a few years now. It is kinda scary and makes me think the trans is on the way out but once it kicks in, it is fine. It does seem like something sticking. I never believe in snake oil but I have heard accounts of people fixing some tranny oddities with Lucas trans conditioner or something like that. If I was going to try some snake oil, which I might, it would be the Lucas. I'm hoping to find a simple answer too.