FINOCJ previously posted:
"maybe I am just an idiot and it was all in a hose connection?"
It'd be GREAT if it was a hose-connection issue - as that's an EASY item to resolve.
As there might be a question if the test was sufficient, I'll drop this thought - the heater core is being tested cold and not at operating temp (thought to a very small hole that only opens up for fluid flow when the metal heats and expands). I put this rather low on the list thought.
How easy is it to remove/replace the core? If time consuming, perhaps leave the core out of its installed location and only connect the heater hoses to it and leave the core, loose, in the engine bay / front fender. You'd be able to run the motor and get it up to operating temp & pressure where you could (with gloves) hold the heater core and turn it around to inspect for leaking.