Incorrect/bad radiator cap or the overflow tank hose(s) is plugged up with crud.

But, you're correct - coolant recovery systems are supposed to work like this. When the engine gets warm coolant gets pushed past the cap into the recovery tank. When the engine cools a vacuum is created that sucks the 'excess' coolant back into the radiator.

If there is a problem, if the line is plugged or the cap doesn't let the coolant back in the vacuum created will collapse the upper and/or lower radiator hose(s).

There are two types of radiator caps - for systems with and without coolant recovery systems. I can't tell you how you'd tell which cap is which, except that some are labeled 'Vented' and these are allegedly for the systems with coolant recovery systems. I went through something similar with my new radiator a month ago - which uses a GM cap - and had to get the cap labeled 'Vented'. IÆve always bought my radiator caps for my Sport from the dealership so I have no idea what you'd look for at a parts store or if the import caps are even labeled.

Edward