A leak down test should enable you to find out where it's leaking from, you say the bores only yielded 5%, how do you know this wasn't going through valves/guides, etc... ? Did you positively identify the bores as being the source of the leak as opposed to inlet/exhaust ?

I believe a leakdown test can tell you if you have inlet, exhaust or piston leaks by listening () in various places while you run the test, come to think of it, it may not be able to tell if guides are leaking...

I would have thought that blocking the inlet manifold, opening the inlet valves and closing the exhaust while pushing air into one cylinder would tell you if any air is getting past the guides.
This might well be a load of drivel though