thanks all for the replies. I'm going to go out and buy a reseating
tool. Any tips on how to use one corectly? My father-in-law and
brother-in-law both tried to fix this problem assuming it was a faulty
washer, so we replaced the entire stem since the old one was the
original, but that didn't fix the problem. I'm pretty certain the
problem is the seat although I do remember my father-in-law feeling
the seat with his finger and proclaiming it fine -but what else can it
be? Trying the reseating tool couldn't hurt could it?

To answer some other questions, I do have access to the back of the
piping though a closet behind the tub but the pipes are cemented to
the wall so a full replacement would entail taking out the wall.