If these are the two old twist knob style faucets then turn off the water at the main (or if you're lucky under the sink); take off the knobs; unscrew the top of the valve; twist out the valve stem (keep track of the number of turns for later); replace the rubber seal on the bottom; replace the two rubber "O" rings on the stem; gently (so as not to scratch the "O" rings) insert the valve stem back in the same number of turns; snuggly tighten the top and reinstall the knobs; turn the water on. When I do this I often have to replace the on/off valves under the sink because they end up leaking when I turn them back on since they are so old.
Kep, April 2004
