Help, my Price Pfister shower pressure balance valve leaks up through the shower head when turned off. I replaced the cartridge and it still leaks. It almost stops leaking right after a shower, but over a couple hours it drips about once every second. If I turn on another faucet in the house (lowering pressure), it starts streaming out the shower head. I have taken the new cartridge out and sanded the back of the copper housing at the o-rings (D-rings?) where the water comes into the housing. I started with 320 grit and then used 600 grit. The front chrome plated part that turns with the handle seemed to be smooth and flat so it could seal with the front o-rings. I also used some AmourAll (silicone) on the o-rings. I put it all back together and still have the problem. Is it possible there is some microscopic corrosion/surface uneveness that stops the rubber from sealing? Do I have any options other than tearing out the tile and replacing the whole valve?

The cartridge can be seen at
Model 0-2085 OEM 974-491