What causes the lower radiator hose to squeeze and flatten the lower hose is flat

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My Mitsubishi lancer 2002 no heat in car only luke warm air it is not overheating top rad. Hose is hot both heater core ho ses are hot bottom rad hose flatened and cool to the touch please help I don't want problems to worsen i don't have any leaks. But when I removed radiator cap it made a sucking sound is that normal