I replaced the elbow in the tentioner and the car temp started overheating fast, on 2000 Chevrolet Monte Carlo

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What cause this to break, when this elbow was replaced less then a year ago because it was rotten.
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Bleed air from system, there is a bleeder near the thermostat. Be sure it is FULL of coolant. NAPA has these elbow's made from aluminum, part # 6601942. They do not rot!!