When I started using the Platypus hydration system (collapsible bladder and hose), I periodically had leaks where water would seep out of the system and get the contents of my pack wet. The source of these leaks was the connection between the hose adapter and the threads of the bladder spout. This continued until I remembered an old trick I first learned in Organic Chemistry Lab and that is also used by plumbers: plumbers tape. Plumbers tape is wrapped around the threads of pipes to form a tighter bond between the male and female connections.
Once I started wrapping the threads of my Platypus bladders with plumbers tape, all leaks stopped and I’ve never had a problem since in hot or cold weather, when the connection shrinks a bit. You simply screw the hose connector to the bladder and tighten it up so it won’t come loose and you are good to go. Plus plumbers tape tape is dirt cheap and you can get it at any home improvement or hardware store.