Originally Posted by
Trsnrtr
Over tightening the valve stem can distort whatever O-ring or gasket is being used. I had a valve that I reused a couple times on different setups and it would never seal. The O-ring
/gasket had become too worn. I swapped in a new valve and everything was good.
Yup, exactly this - It's like overtightening a toilet supply line. It's actually less problematic on the cheaper stems with the conical gaskets that are bonded to the stem; the more expensive stems, usually lightweight machined aluminum with replaceable gaskets, are the ones that you need to pay attention to.