any powerglide (cast iron) Guru's out there?

I have a 57 chevy with the original cast iron powerglide, it works great except it won't shift out of 1st till it's nice and warm. So i either have to let it idle in the driveway for 10min or rev the **** out of it driving around in 1st gear for a good mile or so. I've flushed the fluid hoping for an easy fix but no luck, I'm thinking (hoping) it's still something simple like a sticky valve body but thought I'd ask here before i rip into it too much.

Thanks
Justin (primed55)

 

I used to have a Falcon with a two speed Cruise-o-matic that would do the same thing. Once it was warmed up, it would be ok the rest of the day, as long as I didn't park it and let it cool off too much. It took mine about 25 minutes though. Or about 10 minutes in 1st gear on the highway!

 

Dried out lip seals on the shift servo piston.
Very common......been there & done that.
My 58 Corvette was notorious for that issue.

 

Thanks for the info, so where would i find the shift servo poston?

 

probably under that big side cover. Which has a funky gasket sealing it.