After a decade or more of using mostly canister stoves, I have been messing around with switching to alcohol, so I'm still a bit of a noob in this area. I pretty much have my new stove system pegged now except for one last detail - seemingly the most trivial one - the fuel bottle. I bought a couple of Packafeather caps, the ones with the flip-top opening, a piece of tubing and a gasket. The gasket solves the issue of leaks from the outer screw seal provided contact is made, and I have had no problems sanding down the ends of particular bottles that are a bit too long. The tube also works fine for me in both output and input mode provided the bottle can be squeezed.
The problem lies, at least from my perspective, in that most of the flip tops I have seem to leak through through the closed flip-top opening when put under a slight pressure - this includes both the packafeather ones I just ordered. To be clear, they are leaking through the valve, the outer seal on the screw top is fine. The flip-top valves don't seem to me to be designed very well against this eventuality, which in retrospect seems obvious. Perhaps for them to work requires too high a manufacturing tolerance for these cheap items, and only some will fully seal. I have also tried Sriracha sauce tops (a la Mike Clelland) as I use a ton of that stuff and therefore have a good source of the tops. Those leak as well, though they seem a bit better. I just checked one of the smaller nalgene flip-tops and it seemed fine - I have carried bottles with these smaller ones on them for weeks on trip in the past and they never leaked. Did I just randomly get two crap tops from packafeather that were crappy in exactly the same way (too loose flip-tops)? Or is it something elseI'm not getting?
Frankly I was skeptical of the flip-tops since they pretty much seem designed for bottles that will sit upright all the time. So it seems, at least with these kinds of cheap DIY adapted tops fluid in contact with the "valve" plus a small amount of pressure often equals leaks. In real life the pressure could be caused for a small external shift of things in the pack, or lying the pack on the ground, as well as changes in elevation, though the "leaks" in the last case might actually equalize the pressure quicker than it would change while going up a mountain.
Of course I would always have the fuel bottle in a ziplock, but I'm a bit worried now about major loss of fuel and/or major leaks, not just using the bag as a stopgap against a rare problem with the closure.
How do people deal with this? Are there caps that reliably do not leak under pressure, or am I just going to have to buy a bunch of them and bin them?
Also,It feels like the alternative of switching out the top from a cap to a flip-top every time you want to squirt or suck fuels would partially defeat the purpose. Do any of you folks have the same leakage issue, but just somehow avoid the pressurized situation? I know some people have written about testing them at the store. I couldn't do that with the caps from packafeather, and both were epic fails - the both flip-tops felt way too loose even before I actually checked for leaks.
Perhaps a more interesting question: it would seem there should be some sort of flexible sealant that could be applied to the valves joint, such as something from the plumbing domain, that would create better seals, provided it was not alcohol soluble. Any ideas how the valves could be "fixed" that way?