Sounds like it had an air pocket or was possibly overheated (even though the dash gauge showed it wasn't overheated). You do need a thermostat for proper operation. If you don't have a thermostat installed the water flows too quickly for the radiator to absorb heat properly therefore it can actually over heat without a thermostat. You also need a fan as with no air movement past the radiator it can overheat. You said your dash gauge is only showing half way. That's fine if the gauge is reading correct but rarely does a factory dash gauge read correctly so it may have been a lot hotter than you thought it was. I'd try an aftermarket gauge or laser heat gun to test the accuracy of the dash gauge.