i’m driving a 1986 pontiac sunbird wagon and am having a slight problem with my radiator. the water is leaving the radiator into the overfill tank but not being sucked back into the radiator after the engine cools down. what may be causing this? we just replaced both the radiator cap and the thermostat cap so we know its not those.
You probably have a slight leak somewhere else.
The cooling system is supposed to be closed/sealed. If it were closed, the coolant would be sucked back. As it is, it seems there may be a leak somewhere.
Take it to a shop and have the technician pressure test the system.
Other than that, are you experiencing any other symptoms? (Loss of coolant, overheating, etc.)
the coolant loss all just seems to be into the overfill tank
The siphoning tube back into the radiator from the reservoir tank may be the place where your leak is. As your radiator fluid cools, it draws the resevoir fluid back in, and if there’s a disconnect or a leak, the siphoning is all suck and no payoff.
There’s a joke to be made there. But I won’t. 
However, Beajerry makes a good point, if the connections were loose, it’d suck in air. And in my experience the clamps on such tubes are never really the best.
Check those, tighten replace if needed and if you are still unsure, have it pressure tested.