The first thing to do is to get the device that pressurizes the cooling system. Then, search for a leak..........sometimes it's small and it takes a week to leak out one quart.

If you are positive that no leaks are present, it might be a good time to send off an oil sample to the lab. See if they find glycol traces in the oil. If so, the head gasket is compromised (not uncommon on older vehicles).