Graeme,
The coolant is pumped around the motor, in a never ending loop, and is designed to flow straight through the radiator to dissipate the heat, and back into the loop. In a perfect system, you wouldn't need the expansion tank, as it would just keep looping around.
The coolant itself reaching temperatures near boiling point, causing it to expand around approximately 3% - 4% of its volume, hence the overflow bottle to catch the excess returning fluid.
When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches around 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled.
You will always see the coolant level drop a little once the car is running and the thermostat is open, as the water pump is then dragging fluid through the system....
You may have just opened the radiator cap at a time where the coolant had cooled enough to be drawing back in from the overflow tank in combination with the thermostat being open ( most likely ), you may have a small airlock in the system or your radiator may have a blocked tube/core, causing a little more fluid to go to the expansion tank.
Mic
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