my 02 suby GT 2.5

When the engine cools down...the lower radiator gets sucked flat...like there has been a vacuum drawn...

It's almost like it wants to such coolant back in from the overflow but its blocked (I'm not saying it is...its just that it would seem that way)--it should be sucking from the overflow tank and not collapsing right?

could be low on coolant, 02 legacy could be a head gasket leak

If loosening the radiator cap releases the suction, see http://www.ultimatesubaru.org/forum/showthread.php?t=87072

Start simple, replace the cap, its cheap.

 

nipper

Yup. Check the return hose from the overflow tank isn't blocked then change the radiator cap.

bad cap

 

lev-r-vent it

When you remove the cap it shouldn't have any vacuum though regardless if the hose is collapsing or not?

there will be vac. after the PRESSURE goes away from running as it cools