As they say, check at the pressure of the cooling system. It can be something as simple but very common as the refrigerant tank cap does not work. This plug opens when the pressure of 20.3 psi reaches. When it is no longer useful, it opens at lower pressure and therefore the water of the system boils. Some caps that are sold do not even indicate what pressure they open. You can try the cap, connecting a pressure gauge where the refrigerant enters the tank and where air is injected using a simple air pump (the bicycle). Go until pressure the cap opens and starts to let out.