So if you do have a failed head gasket and small amount of exhaust in the coolant system, it will push water into the reservoir and probably over fill it and loose water. Ignore it long enough and it will run hot eventually. Testing for this isn't difficult..
And for the record, the copper colored head gasket fix from Barsleak really works. It fixed my 96 camry 4 cyl. well over 7000 miles ago.
