My 2004 impala is idling when I turn it on & I noticed the radiator hose is flat like air being sucked out?

if that is just at the start, then the cap could be at fault. the valve that lets fluid back into the system when it cools down may be stuck closed and as the water condenses, the hoses collapse due to a lower volume. if that hose stays that way even when it is hot, id feel the hose and see how stiff the walls are. i have to assume that you are talking about the upper hose.