If your vehicle is showing upper radiator hose is sucked flat or collapsed when the engine cools down, here are the most likely causes and the parts you'll need to fix each one.
The radiator cap has a built-in vacuum relief valve that allows coolant to return from the reservoir as the engine cools. If this valve sticks closed, the cooling system creates a vacuum that sucks the upper hose flat.
Installing a non-vented cap (designed for older open systems) on a modern coolant recovery system prevents proper vacuum relief during cool-down. This is a frequent mistake after recent cooling system maintenance.
Over time, the rubber and internal reinforcement of the radiator hose break down. A weakened hose loses its structural integrity and will collapse easily, even under normal, slight system vacuum.
The small hose connecting the radiator neck to the coolant reservoir can become clogged with sludge or debris. This prevents coolant from returning to the radiator as it cools, creating a vacuum.
Sediment or cooling system sealant can build up at the bottom of the coolant reservoir, blocking the port where coolant is supposed to be drawn back into the radiator during cool-down.