There is a little o-ring inside the female side, if it gets nicked the QD will leak. It can be replaced, easy to pull with a pick but real fiddly to install the new one. I can't remember the size but I measured mine with calibers years ago and order a bunch from the the O-ring store, for like $10 bucks shipped. If I remember (old timers) I'll look in the shop to see if I have the size written down.