a bloke i know has the same problem as you, he kept on topping up the coolant level but it keeps spitting it back out of the expansion tank vent, he got a mechanic to look at it and he couldn't find anything wrong with the system. as the car doesn't exhibit any cooling problems and no sludging under the oil filler cap, i think he has just accepted the fact that the coolant seems to find its own level, which is just under the moulding line on the expansion tank.