I have a hot water shower faucet that is leaking only when I turn on the shower. It is leaking at the o-ring where the stem screws into the pipe. The stem is a Price Pfister 5084: http://www.idealtruevalue.com/servlet/the-72661/Detail
I have replaced it with a new Price Pfister 5084 stem, but it's leaking like before. Then I put one more o-ring before the o-ring on the stem and screw the stem into the pipe. It no longer leaks where the stem screws into the pipe, but starts to leak around the white stopper when the shower is on.
What could be the problem with this?