To change the washer in a bathroom shut-off valve, turn off the water at the main shutoff point, disassemble the shutoff valve in the bathroom, and replace the neoprene washers. When disassembling the leaky valve, do not disconnect it from the water supply line or the line leading to toilet.
At the main shutoff, turn off the water to the house. Disassemble the leaky valve without disconnecting it from the connected lines. Turn the valve knob and packing nut until they are loose and can be removed. When using pliers, cover the jaws with masking or cloth tape to protect the components. Replace the neoprene washers on either end of the valve stem. Reassemble the valve, and turn the water back on at the main. If the valve continues to leak, the entire shutoff valve assembly needs to be replaced.
Shutoff valves in the bathroom have neoprene washers that can become brittle or crusty from years of inactivity, and they develop leaks as a result. Before replacing the washer, try tightening the packing nut by one-eighth of a turn with groove joint pliers. The gentle movement may be enough to restore the washer and the seal. The valve can also be disassembled, and the washer cleaned with a cloth and flexed to make it supple again.