what does it mean when you pull the radiator cap and find this
brown colored sludge around the neck . Is it manifold gaskets , head
gaskets, cracked head, or a bad radiator leaking trany fluid! This is a
3.4 engine in an Impala.

time to flush/replace the coolant....dexacool strikes again

Some of this seperation can be normal in small amounts on the cap. Look down in the radiator cores to be sure and change the cap also, any poor sealing of the cap can also cause this.

max: good call.

Why I could never open up the radiator cap on my impala? turned cap to the release position, then pull whatever so hard, just can't...

yeah, I thought I was doing wrong too, just run it until it stops, then you have to push down and continue to turn it, thats how you have to take off allradiator caps, except the impala one, is really really hard to puch down far enough to finish turning it off, I had to use two hands and some body weight.

with the advice I finally managed to open up the cap. Well.... sludge!!! but not much, about 1/8" thick inside the cap and around the neck. It's pretty oilly. As far as I know the dexcool is much like oil, right?

Anyway I'm going to get a coolant flush with upcoming major maintainence. Had head gasket replaced about a year ago. They just mentioned "changed milky oil", but not coolant.

Were they supposed to do something to the coolant side in the common "head gasket" cases?

Normally the sludge you refer to at and around the radiator fill neck is remants of the coolant system sealant that is put in at the factory when new. It is similar to the kind you might find at your local parts store and is normal in small amounts around the filler neck. Wipe out neck and either flush or top off. Otherwise if your coolant is totally contaminated a blown intake gasket is likely.

you could be right for mine. The "sludge" is much like old deposit of plumber sealant I saw when taking apart household drain pipes.