If the leakdown test is inconclusive (which it may if the leak is small enough)the best method I've used is with a smog machine sniffer in the radiator neck. Any amount of combustion gas will be detected that way even if it is slight. No access to such a method? There is also the block check kits (available over the counter) which use a blue liquid that when exposed to combustion gases it turns yellow. Very easy to do if done correctly.
Hope you figure it out.
