When doing repairs or checking the fit of plugs, pads, and cork, we use a simple test in the workshop to check for leaks and ensure that the various parts of a flute are airtight. The good news is, you don't need to be a skilled repair tech to do this at home, and anyone can learn to diagnose a leaky flute part.
You can do this test on any section of a flute, and while we use it most often to check a head-joint for leaks, it also works to check for leaking pads.
It's best to check each piece separately. For instance, to check a headjoint, remove the barrel and hold the head with a thumb or forefinger over the embouchure and place your lips on the metal tube end. Draw air out of the tube to create a partial vacuum, and keep the seal with your lips or tongue. You should be able to feel the resulting pressure as air tries to get back into the tube. After a few seconds, remove your lips, and there should be a slight "pop" as the pressure equalizes. If the tube is leaking, you won't be able to maintain this vacuum, or it won't hold for long and you'll feel the pressure reduce as air passes back into the head through the leak.
It's possible for a head to leak either around the plug or between the metal of the head tube and the wood of the headjoint, so if you detect a leak, the next step is to ascertain where the leak is coming from. To do this, remove the end cap and cover the end of the head tube with the palm of your hand so that the end is sealed, keep the mouth hole covered as well, and repeat the above suction test. If the leak is no longer detectable, it's likely that simply replacing the plug will solve the problem. If on the other hand, the leak is still present, a more delicate repair to seal the head tube may be needed. Our plugs (which are carefully fitted and faced with sheet cork) are very unlikely to leak so if a head is leaking, there's a good chance that the latter situation is to blame, with air leaking between the tube and wood of the head.
To check a midsection or footjoint for leaking pads, it is necessary to cover all the holes and repeat the suction test as above. With a one-piece midsection this is a little tricky, as you'll need both hands to cover the finger holes, so you'll have to either enlist a friend to help by covering the bottom of the midsection, push it against the bare skin of your leg to seal the end, or seal it in some other way. If you find a leak, it's probable that one of the key pads is not sealing properly and may need to be replaced, or that one of the springs needs to be adjusted.
Related Notes on Oiling:
It is possible for congealed oil and dust to get into a keyway (especially if you're over-oiling your flute) and cause the key to bind in the slot so that it is not able to freely return to a closed position as it should, which can cause a leak. It's a good idea to familiarize yourself with the action of the keys so that you can feel the difference when a key is binding in the slot.
When oiling a flute—generally inside only and no more often than needed—it's important not to over-do it. It's best to do this before playing the flute rather than after, so that the bore isn't saturated with moisture from playing. Aim for a light coat of oil that is not enough to run down the bore or cause drips. If you sight down the bore against a bright light, it should look shiny after oiling (in contrast to the dull, dry, matte look before oiling). Let it sit for a few minutes, then use a dry rag or paper towel on a cleaning rod to swab out any excess that could build up on the bore and the recesses that the keypads hit. We recommend almond or olive oil rather than bore oil. Petroleum-based bore oil is supposed to be non-hardening, but we have had some customers complain that it has built up on the bore and gummed up the keys and/or pads.
Here's an example of a flute that has been over-oiled and the keys not removed or protected from the oil:
You can see above how the oil has built up on the pad, interfering with the sealing action of the key, which is why is is a good idea to remove the keys when oiling, or to protect them with a twist of plastic wrap until you've allowed the oil to soak into the bore and then swabbed out the excess:
We hope this description will be helpful in explaining the process of a "vacuum test" and enable you to check your flute for leaks if something doesn't feel right. Happy fluting, and as always, drop us a line if you have a question!