I have a older Price Phister two handle bath tub faucet that does not shut off the water supply. It is an older house that the water was shut off the last two years. I have pulled out both stems and replaced the seat washers. When I turn off either handle the water supply decreases somewhat but still flows substantially. When I look at the seat, I do not see a way to remove it and there is no access panel. Should I replace both stems first?