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Coolant wont flow with the pump not running and then as the temp rises any air turns locked in anywhere in the system will to steam and blow out the coolant.
If you can get a pressure tester and...

No all it will do is cause an air blockage and the temp rises quickly and then blows out the coolant.

Could even just be something as simple as a stuck thermostat creating a blockage.
Or water pump vanes have fallen off and coolant isn't flowing properly. From memory some JD pumps use a plastic...