They get old and become weak.
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radiator hose needs replacing. use and heat will cause hose to collapse when worn out
A water thermostat stuck open. Air in the heater hose lines (caused by low radiator coolant). Air temperature (mechanical) control cable not opening the heater hose water valve (under hood).
Remove the bottom radiator hose. Put a garden hose into the top of the radiator. Allow the garden hose to run until the water coming out of the bottom of the radiator is clear.
Lower radiator hose. It may be easier to use the drain plug located on the left hand side of the radiator. Its on the back of the radiator. Do not mistake the heater core for the radiator. The radiator is the one with the thick rubber hoses. To take off the lower radiator hose you have to remove the air filter box.
If your truck is overheating!!! Then check the upper radiator and lower radiator hose's , if the lower is cooler the the upper the either the thermostat is not opening or the radiator is clogged