The purpose for the coolant reservoir is for expansion, when at operating temperature the water expands and have some go into the reservoir, When the engine cools coolant /water condenses and draws the fluid back in, in you case you are loosing it, this is mainly due to a leak, means you need a pressure test to find the leak. This can be at a number of places, the waterpump, radiator, heatercore, cooling hoses
March, 8, 2013 AT 6:47 PM
As he said, get a pressure tester from the local parts store and when the engine is cold, pressure test the system. Once it is pressurized, then you need to look for leaks.