I did some serious leak checking, and found that the barrel is not air tight (and I also found a couple of leaky pads…). It has a repaired crack (by Jon C.) but somehow I have the feeling that this might not be the problem. Perhaps it’s leaking from the barrel’s metal lining…? If this is the case, what could I do?
Come on, I’m pretty sure Jem’s not the only one that can answer this question!
I’d pull the headjoint off and do a suction test. Close of the bottom of it, suck in through the embouchure hole and when you pull your lips away there should be a slight popping sound. If you’re not getting that, then yes there is a probably leak somewhere. I have had leaks that were just of matter of a headjoint that was too loose, so I had it expanded and that fixed the problem. I doubt the lining of the barrel is leaking, but you could always pull the headjoint out of it and make sure this section is airtight too. In general when looking for leaks you have to just check one section of the flute at a time.
If there is enough slack in the slide it can act as if there is a leak.
You can suck test both parts and then do it together.
I have this exact problem with my Wlyde.
I make a bee’s wax / Jojoba oil thread/cork grease that sets up rather stiffly. I just make sure the slide has enough and it remains air tight.
I imagine any cork grease not to thin should work just as well.
Give it a try.
I’ve already said that it is the barrel alone that is not airtight. The headjoint is fine, I’ve checked them separately, and the tuning slide should also be fine since i waxed it, but I can’t check this properly untill the barrel section alone is airtight.
I only see two options for the leaky barrel: the repaired crack (but it looks well repaired) and the metal lining (maybe some air passes between the metal and the wood?). The question was, what to do in the second case? Maybe I could put something where wood and metal connect, like some wax or shellac or glue?
I take it from your OP, Lorenzo, that you have checked the barrel separately from the head and that the airtightness of the slide is not the issue?
So, if there is a leak in the barrel alone, either the old crack has reopened or there is a new one or there is a leak between the liner tube and the wood. You can check for the latter by putting some spit around where the protruding metal part comes out of the wood, stop that end up against your hand, then put your lips around the socket end and blow - see if you get any bubbles. Do the same along the old crack line too. Let us know what you find.
Ah, cross posted. Thought as much!
You’re on the right track for solutions if it is a leak between the liner and the wood. This is what I’d do once that diagnosis is confirmed. I’d run a line of superglue around where I said to put the spit, then suck at the socket to draw it in. Then wipe away any excess and clean up with acetone. Let it set, then re-test and repeat if necessary. Then you could also run some glue round the inner end of the liner tube on the little shelf at the bore step out to the socket.
Typos corrected.
I knew Jem was the man! Yep, bubbles, my feeling was right 
So, superglue, sounds a bit scary…
As long as you have some acetone and rags to hand and take care where you let the glue go, it’s not a problem. I usually put several drips of glue in a little puddle and then tip and/or blow it around where I want it. Of course, you have to be fairly quick. Removing excess with acetone is easy enough. In a case like this where you want to suck it in, you need to put a fairly liberal run of glue right round the point where the metal goes into the wood. Keep it upright, lift it up without tipping it and get it above your face to suck at the other end. Don’t suck too hard at first just in case the glue can get right through too quickly and into your mouth!
Perhaps the head of a turkey baster might be a better object to do the sucking. Squeeze, then release.
http://www.pantrymagic.co.nz/WebRoot/ecdb8/Shops/pantrymagic/492B/A020/AF8F/5CB2/7DCD/C0A8/D218/FF6D/BT-WY25-007856_m.jpg
Mmmm. Is Lorenzo’s silence due to having a flute barrel stuck to his lips? 
Now I’m at a computer and can write more easily than on a mobile phone… I just want to add that things depend a bit on how much of a leak there is. If Jon (as I presume he would have) removed the barrel liner and glued it back in after fixing the crack, either his glue didn’t effect a good, complete seal or it has given up since for some reason. If you have a very large leak, then sucking superglue into it may not be adequate to seal it up properly and it may be necessary to consider again removing the liner tube, cleaning up old glue residues and refixing it. If the leak is minor, the sucking in of superglue and running superglue around the wood/metal interfaces at each end will probably do a very adequate job. To remove excess glue, I would hold an acetone-soaked cloth over the glue for a while, then flake away any large bits with a fingernail or even very carefully scrape it away with a knife blade (taking great care not to scratch wood or metal), then rub any small bits remaining away with more acetone on the cloth. This ought not to destroy any newly achieved seal of your leak as the glue effecting that should be inside the contact area between wood and metal, not just a bead in the crack at the outside.
Sorry, I didn’t have superglue at home (well, I had it but it dried in its container…) so I had to go buy some. I’ll seal the leak tomorrow
Oh, and the leak is surely minor…
Done,
It is airtight now (i didn’t remove the excess glue yet). Thanks guys 
Oh, I missed the drama!
The liner was glued in with epoxy, strange that it developed a leak…
Glad it worked out for you!
Now I will go back to sulking in my room… 
Fair play Jem!
Ah, so that’s where you were/what you were up to… 
And I thought I was a bit late to the table. 
Carry on…