The closed cooling system which as already mentioned refills the block and radiator from the coolant reservoir as the engine cools, providing everything is tight and the vacuum valve in the cap works. Over my many years working fleet maintenance I took a liking to the spring type hose clamps. I felt they could expand and contract on the hose as the necks of what ever grew or shrank according to temerature. I began to have a heater problem as you discribed this fall. Every time I would check the coolant it was down in the radiator maybe a half a cup. I looked for leaks and began to really think about the
problem. Since we need vacuum (low pressure in the radiator) to pull the coolant back which has been pushed out to the reservoir during the heating cycle I knew I had a leak somewhere. I found it apparently the spring type clamps on the upper hose had relaxed
enough to seep a very slight bit when cold. Replaced and the problem is gone. My suggestion is check all your connections and get the system tight before replacing anything else. kotzy