If you went through multiple valves like that then cooling system pressure was likely going too high too often, such as a failing head gasket or a bad rad cap. The original plastic and metal OEM heater valves were known for breaking with the plastic getting brittle with age, becoming the weakest failure point in the cooling system. The result was overheating from a sudden loss of coolant. The new designs are all plastic and seem to last. Be glad it was heater valves failing and not the heater core -one of the worst 700/900 jobs going.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now