[MikeN]

I'm wondering if your radiator is plugged with a wasp's nest or something. IF your engine never fired, the only way to build pressure in the system is if the water pump is pumping (when you were turning the engine over with the starter) but the radiator is plugged tight.

I'd check the heater core and radiator both for being plugged.

But I'm trying to think this through - when the engine is cool and the thermostat is closed, coolant circulates without ever going through the radiator - I just can't imagine how you are building pressure if the engine doesn't fire.