So I thought I have a heater core leaking. Maybe I do, but after bypassing the heater, the coolant disappearing problem was still there. Turns out to be more related to intake gaskets after all. While making this determination the heater hoses were removed from the heater core and capped off to eliminate the core as the source of the leak. Now it has new intake gaskets. The shop doing the work says I can't run the truck with the heater hoses capped off, they must be connected so water will circulate through them. ??? I have to wonder what's so different about this engine's cooling system that water MUST circulate from one port to other? Cars, trucks, (anything with a heater) have used heater valves of one form or another for years to stop water flow through the heater when heat is not required. Stopping that circulation has never been a problem, so why is this system different?