plumbing supply shutoff valve behind toilet or under sink can go for years without being turned off and then reopened again. inactivity can cause the packing washers to become either brittle or crusted up with mineral deposits, and they lose their ability to seal leaks. sometimes, after shutting off a toilet supply valve to replace the tank plumbing parts, you may find that the shutoff valve leaks through the packing nut once the valve is opened again. To remedy this, most plumbing pros advise first using a groove joint pliers to tighten the packing nut one-eighth turn clockwise (see video). If the packing washers are still flexible, this should stop the leak. If tit still leaks, before replacing the entire valve, try watching this video