Ding, ding, ding! We have a winner!

If it's the suction hose (the one that goes to the bottom of the radiator) and it flattens out while the engine runs, it should be replaced. If it's not the suction hose then you look for what's causing it to suck flat. My money would be on the radiator cap and that the hose is normal when the engine is up to temp but only goes flat after it cools off.

When the coolant gets hot it expands and pushes past the radiator cap to the overflow tank. If the cap won't allow the coolant to return as the system cools and the coolant contracts, you get a vacuum in the system.