top radiator hose vacuumed in?

this is on my dad's car.. its a truck. i'd post in the toyota forum but nobody responds. anyway the top main hose gets vacuumed flat on a warm engine. like no air in the hose. when the radiator cap is removed, it expands normally and is shaped like a circle againagain. there are no heating problems, the thermostat isn't stuck. i think it needs a new radiator cap. what else could it be?

fill up the coolant and replace the rad cap.