As far as I know it has had this problem since i bought it, but I haven't driven it that much. If you drive with the heater off it behaves normally, and you can pressurize the cooling system with a pressure tester tool with the engine off and it holds pressure for at least half an hour, but with the heater on you can drive until the engine has has more than enough time to get up to operating temperature and open the hood and squeeze the upper hose and there is no pressure at all, then turn the heater off and drive a bit more and it has the normal amount of pressure it it. There are no visible leaks, it doesn't loose coolant, and no signs of coolant or oil ending up where they don't belong. The thermostat, coolant reservoir cap and radiator hoses are new, the rest of the cooling system hasn't been touched. Any ideas?