My experience is that a leak-down test will only show a problem if the static seal on the seat is poor. If it is a dynamic problem as it was in my engine then the leak-down doesn't help. In my case the valve was deformed in such a way that the seat mating surface was OK but the rest of the valve and stem had effectively stretched. When the engine was running the valve open time was probably wrong leading to rough running.
I didn't check the compression using a standard compression tester whilst cranking the engine, that may shown the problem. Measuring the stem length would have definitely shown the problem. I think that mine was a rare problem, hopefully this one will be more straightforward to find.
Kevin
