the bottom oil ring shouldnt have anything to do with a leak down test. you could take the bottom ring off of the piston and your leak down test would be the same. the top 2 rings does all of the sealing work. if air is getting into the pan then you have the wrong pistons, rings, or rings on wrong, or the block is to big for the piston. i think you're putting the top rings on up side down. you ned to check that close.