After years of perfect service the 2 nd floor bathroom faucet started leaking into the cabinet below! Fortunately we had a large plastic tub holding all the stuff that goes under a skink which also caught and held the dripping water. Finally determined the water leaks rapidly whenever either the hot or cold water is on at the faucet. Found the faucet was leaking at the base of the water spout. There were two screws holding the spout to the main assembly and an 0-ring there trying to seal the joint. This o-ring must have shrunk over the years as it was just too skinny.
I just happened to have two o-rings of similar size and installed the biggest and all is sealed up tight. (The removed o-ring measured approximately 0.581 inches ID and 0.737 OD. The replacement was 0.562 ID and 0.764 OD.) The replacement had just a little friction when installing the stem which means it will seal much better.) Now no leak! Maybe good for 10 more years.