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With leaks from the pipe in the valley it wouldnt be that severe as to leak a whole gallon in 10 days! The ones we have seen are just minor seepage and maybe light puddling in the block valley.
Your comment about the valve guide seals got me thinking, if the intake gasket is leaking above that questionable cylinder, could coolant be making it's way not only into the valley, but dumping some into the intake port of that cylinder. Apparently anything above 150k and the gaskets get brittle, and that's when the fun starts.
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Have you considered this tool?
https://www.amazon.com/Block-Tester-BT-500-Combustion-Leak/dp/B06VVBSFTF