the problem is that the pepsi, coke, ect. bottles dont get a very tight seal after the first time their opened kiwi because thats all the need to do
so im assuming thats why sayworth bought diffrent caps that maybe were supposed to fix this problem
i agree that it sounds like a leak in the cap but switching to the one it came with wont help
one solution is something like the the sigg waterbottle they cost more than a pop bottle but they are way tougher and seal better (when i go backpacking with my sigg water bottle sometimes will hiss when I open it because of the elevation change)
"A SIGG Bottle is your healthy and safe choice for your body. The baked-on, crack resistant bottle liner makes sure that you drink what you want to drink – and not unwanted chemicals. The SIGG bottle liner is leach-proof and resistant even to energy drinks, fruit acids and carbonation." http://www.sigg.com/sustainability/healthy-safe/
I have used 2 sigg bottles over the last 5 years(five very tough years) one got too dented and the interior chemical treatment started chipping off i discontinued its use but was not disappointed it denting after falling 5 to 20 meters is to be expected.
also I've used it to store hot hotchocolate, hot tea, iced tea(all the time), and water of course. The only part that takes on the flavor is the lid and thats replaceable. ive found it much better than a reusable plastic bottle and far better than a single use bottle.
you can reach me at firstname.lastname@example.org
ps i would love to see a picture of the caps you used (if they were different than the standard plastic bottle caps)