We have seen problems caused by the Schrader valve core becoming partially unseated (or not properly seated during initial assembly) inside the female hose fitting; that's corrected by using a Schrader Valve Tool to tighten the core back in place. We've also seen slow leaks resolved by replacing the worn 011 O-ring located inside the barrel of the female hose fitting, although replacing that O-ring in the QD is challenging to do without a QD O-ring insertion tool and most consumers just replace the entire hose.