[rotorwrench]

Non pressurized systems like the Model A & B 4-cylinders loved to blow coolant out when they are under stress. Coolant expands as it gets hotter. You can check for coolant pressure problems by just letting it warm up at idle and watching the coolant in the radiator. If it bubbles up and blows foam than there likely is a compression leak into the water jacket. It should be just flow. The fan belt can be removed too to see if it is blowing bubbles with no water pump in action although it will boil if the temp gets up high enough. The Model T didn't use a pump at all since it was designed for the thermosyphon cooling principle.

Getting a good seal with the radiator cap and installing one of Skip's 4 psi pressure valves may help. Sometimes the addition of a catch bottle or overflow tank will do the trick on keeping the coolant from going overboard.