My first thought, reading your explanation was, "I wonder if there's a spring in the lower radiator hose to keep it from sucking shut as it gets hot and the rubber gets softer?"
BTW, with no thermostat the coolant can flow so fast that it doesn't have time to lose heat in the radiator. Some old school guys used to run a restrictor in place of the 'stat, a big washer with the hole sized to drop the flow where it worked. A good 'stat is better, and the right way.