Another thing you can try is to take the keys off and use blu-tack or some other similar reusable putty, to seal all of the keyed (closed) holes.
Be careful not to push it too far into the hole – you don’t want it protruding into the bore. And don’t seal off the open keyed holes on the foot
at this stage.
Once you have done this play the flute and see if it is noticeably more resonant. I often find that older keyed flutes play dramatically better
when I do this. Knowing this gives me incentive to diligently work through the keys, one by one, to find out if the flute plays better or worse
when I add that key back instead of the blu-tack.
I find that it is quite common for a flute to pass the suck test, but still have a leak. As you pointed out, suction helps to close a leaky pad, whereas
playing the flute generally introduces some internal pressure oscillations that will push a borderline leaky pad open, especially at the locations in the
bore that correspond to pressure antinodes for the note being played. These are the points in the bore that experience large pressure changes. On
many D flutes the c key is fairly close to the location of the pressure antinode for the bottom D, so it is often a culprit if that note is weak, as Bob
pointed out). The harder you push the flute the higher the maximum pressure at these points and the more likely you are to cause a weak key to
leak and weaken the note being played.
Leaking is often due to either a deteriorated pad which needs replacing (ideally with a high quality, thin, leather clarinet pad), a poorly seated pad,
which needs re-seating (ideally by floating it back into place using heat), a bent key, which needs straightening, or a weak spring, which needs
repositioning/bending or replacing. Of course, there are other reasons for leaks, such as cracks or damage to the tone hole rim on which the pad
sits, but the former are the most common key-related reasons for leaks in my experience.
It is definitely worth spending time to trouble shoot these issues. It can greatly improve the performance of a flute.
Oh, and to the specific question asked, I don’t think an LED strip would really help. The standard, non-technological approaches will allow you to
track down the problem 99% of the time … if you spend enough time and are careful about it.
Then again, now that I’ve hopped on the keyed flute train, I wouldn’t want to go back. Just part of the learning process, and thanks for the help.