Video set to private. At any rate, when I first got my 64 Valiant, it did very similar. As soon as the thermostat opened and with the cap off, when I revved the engine, it would blast out of the radiator filler neck. If that's what yours does, I would have the radiator checked for flow. Mine was stopped up, giving the coolant coming into the top tank from the engine nowhere to go. Just because something is assumed good, means nothing. Keep that in mind. As soon as you say, "the radiator's good" you're setting yourself up. Keep an open mind.