Mr. nickleye is right. Thats why the needle goes down 3 notches and then back up, thats when it hits an air pocket.
Your symptoms scream BHG. If you are clearly losing coolant and gaining air in your cooling system, and its not leaking on the ground then the only thing it can be is the head, block or the head gasket. You should be praying its only the head gasket. I've seen cracked blocks on multiple cylinders before.
Did you remove the overflow hose from the nipple on the radiator and plug the nipple when you did your block test? If not, then you sucked outside fresh air into the tester giving you a false negative reading.
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