A leaking water shut-off valve that does not respond to tightening most likely needs new washers. Minerals within your water can affect the rubber washers within the valve, preventing them from creating a water tight seal. Replace the washers within the water shut-off valve, and you should stop the leak as well.
Also, if you find that you often must replace washers and other items you may have particularly hard water. A water softener will prevent further mineral deposit damage, as well as make it easier on your fixtures and other items that require water to work.
Step 1: Turn off the main water supply to prevent water from continuing to leak. Relieve the pressure in the pipes by turning on the faucet closest to the water shut-off valve.
Step 2: Remove the handle of the water shut-off valve with a screwdriver. Set aside the handle and the screw to replace after you have replaced the washers.
Step 3: Loosen the nut on the water shut-off valve. This is called the packing nut: you will need an adjustable wrench to loosen it in most cases.
Step 4: Pull out the valve stem from the water shut-off valve. This should expose the rubber washers- in this case they are called o-rings. Use these o-rings as a guide for locating suitable replacement o-rings.
Step 5: Remove the o-rings and clean the stem thoroughly with a toothbrush and white vinegar to remove any mineral deposits. You may have to soak the stem in white vinegar for 20 minutes in order to remove any problems.
Step 6: Add the new o-rings, placing them in the exact location where the old o-rings once were.
Step 7: Slide the valve stem into place and tighten the packing nut to keep it into place.
Step 8: Replace the water shut-off valve handle using the screw that you removed in the second step.
Step 9: Turn on the water supply. Look for any leaks and try tightening the packing nut with the adjustable wrench.
TIP: If the unit continues to leak, you will have to replace the entire water shut-off valve.