expansion tank q's
 

i flushed my radiator yesterday, and was wondering whether the expansion tank allows the coolant to go back into my radiator, or if it just hangs out there. it had green coolant in it and i flushed all that out and put dexcool back in it, if it starts sludgin back up, i'll switch back. its a 3.8 98 firebird. there wasnt anything in the expansion tank, and the radiator was low, so i was wonderin if i should worry about anything stirring up any old green stuff and sludgin up my system. also, i checked the coolant and it looked like a little tiny bit of green might have still been in it, floating very thinly on the surface. would that affect the dex cool, and if so, how long can i wait to flush it back out again. thanks