[biggestjohn]

It sounds like thermal expansion is opening up a leak when engine is hot. I think that I would hook up my pressure tester, then warm up the engine. You should be able then to keep pressure in the system while you search for the leak.

Don't forget the freeze plugs in the rear of block.

I would also check the torque on the water pump, manifold, and thermostat housing.

If that doesn't show where the leak is, you will have to check for cracks in the heads. Do a cylinder pressure test, or better yet hook up a cylinder leak down tester.

It has to be going somewhere, and being a sealed system, I don't buy the burning off theory. Straight water is a better cooler than a water\antifreeze mixture.

My 2 cents.