i have a leaking faucet from the tub spout cold side. i took the valve stem out replaced the seat replaced the beveled washer but still leaks .dont know the brand but the the stem is 12in long the seat washer sits in a round cup the seat washer color was yellow. i took it to a plumbing store he suggested the beveled washer was sitting too deep so he put a black washer to raise the beveled washer but it still leaking any ideas? if i shut the cold water at the main valve no dripping but also no cold water in the bathroom. i assume the unit is 20/30 yrs old.