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I don't get your description of the Nibco valve. Is it a shower valve? a wall hydrant? something else?
You mentioned "dressing the seats". That could be the point where the water is seeping by. Unless you have a model number for your valve there's no place online that will be able to provide replacement parts. Using anything other than OEM part seldom works. You could try calling http://www.chicagofaucetshoppe.com/
If these guys can't get it then it's not available. Perhaps replacing the valve is the best option. If you're concerned about messing with tile on the wall then you can use a cover plate to make a big enough hole to work in. Then the plate covers the hole. We routinely replace valves using this method. You're probably better off with a new pressure balancing valve anyway. See attached for a pic of the cover plate
I'm quite familiar with 2 handle tub/shower valves I've literally replaced hundreds of them with a new pressure balance valve. My reference to a pressure balancing valve was for a complete replacement. You'd have to cut out the old valve and pipe in a new one. And like I said before you make a hole a bit smaller than the cover plate and work through that hole. It's plenty of room. And like I said before Chicago faucet is your best bet for parts.
But if this was my call I wouldn't waste any more time on this piece of junk Nibco valve
For standard replacements we use American StandardR120SS
With Colony Trim
This valve has a ceramic cartridge and lifetime warranty form American Standard against leaks.If you go to a plumbing supply house that sells American Standard you can often get this as a package. And a supply house is your best bet for a cover plate
You're welcome! I'd appreciate a favorable rating