LA 360 Internal Coolant Leak

I left the coolant pressure tester for a good while and coolant level did not seem to go down even though the tester was losing pressure every time. I pulled my newly installed plugs and they are black. Doing a compression test now. Also, going to attempt to bypass the old MSD box to see if that may be causing an issue. I've heard some bad things about those boxes causing intermittent issues.