Originally Posted by gran touring
I would bet it the cap shot or a pinhole leak in a hose it easier to suck air than a liquid.
A) The cooling system is pressurized, it doesn't suck, it blows.
B) The centrifugal pumps can not compress or pump air.
If there was a leak, it would lose fluid until the water level drops below the point at which it can reach the pumps intake. At which point the pump the will cavitate and just spin doing nothing. The now still water in the system will heat up, expand, and boil which increases pressure and water will again begin flowing out the hole and most likely exceed the radiator cap relief pressure, blowing off coolant and steam into the reserve tank. The drop in pressure in the reserve tank will cause it to immediately boil the liquid coolant. Futher coolant volume will be lost from the reserve tank overflowing and the coolant converting to steam. Then you pull over and stop the engine.
Once the remaining coolant cools down from the overheat, it will have reduced in volume due to the leak, overflowing the tank, and conversion to steam. Without the leak, as it cools, pressure would drop and it would suck excess coolant back in from the reserve tank. but since there is a leak, no drop in pressure will occur and nothing will be sucked back in. Coolant levels will remain critically low until you refill it and fix the leak.
For those curious, yes I'm a geek.