Originally Posted by Pleiades

The fact it's boiling and leaking from the reservoir, which not part of the pressurised part of the system, would strongly suggest the the cooling system isn't retaining pressure. With no system pressure, the coolant will boil at a much lower temperature and also a no pressure will allow the expansion tank to fill as soon as the motor fires up and the coolant starts expanding and eventually overflow and the coolant will evaporate - be lost. This lost coolant (in the expansion tank/reservoir) will never get sucked back into the cooling system if there is no system pressure because that pressure is required to produce a vacuum as it coolant drops back to ambient temperature when the motor is stopped. Effectively what's happening is every time your engine warms up coolant is pushed out by expansion, but each time the motor cools it doesn't return, the net result being a lower and lower coolant level (and premature boiling).