The hot water supply line brass seat fitting leaks where it enters into the concave section of the Pfister mixer valve for a tub/shower faucet. I have cleaned both surfaces with emery cloth and wire brush, tried wrapping with teflon tape, also tried pipe thread compound, but it still leaks. The faucet is about 20 years old. Looks like it has been dripping on and off for some time. I have pictures of the valve and fitting I can send. I even heated up and removed sweated male fitting the brass valve seat threads onto and made sure everything was square and resoldered that joint. I am wondering if there is a tiny crack in the valve seat. Any ideas?
Don't know about that particular model but I have brass seats as well. I had a leak and carefully replaced washers on both sides properly. Still had a small leak. Replaced both valve seats (only $4 each so why not) even though I knew it was clearly the cold leaking. Checked and double checked and examined seats and washers carefully -- nothing odd. Still leaked. Finally cut some teflon tape lengthwise to match the width of the threads on the seat and wrapped kinda taught 2.5 times around. Put them back in and torqued fairly firm but not overly hard. Leak was fixed.
Brass seats are often fussy. Don't use any tape or other compounds. They are designed to be bare metal to bare metal only. The brass is slightly malleable to allow the faces to conform to each other.
Take it apart, clean and polish the faces with fine grit (200–400 grit) emery cloth (or well worn sandpaper so that it doesn't have structural wrinkles which cause systematic scratches or grooves) until the surfaces are a consistent haze with no visible scratches. If you can see visible striations in one direction, polish perpendicular to that until invisible.
Assemble the parts loosely and gently tighten for snugness, but not tight. With a drip pan underneath, apply water pressure. Slightly tighten (1/8 turn) until two consecutive turns do not decrease the drip rate. Remove water pressure (optional) and let it be undisturbed for at least an hour. Reapply water pressure. If the drip rate has not decreased substantially, slightly tighten once more and wait an hour.
If the drip has not completely stopped after three repetitions, take it apart and check the evenness of the polished faces. Perhaps there is a non-spherical or pitting defect. Determined work with emery cloth can fix small defects, but a large mismatch requires so much work it is probably cheaper to replace the parts.
Over-tightening a brass fitting is guaranteed to make it leak. At torques common for steel fittings, brass fittings tend to split or crack. Use restraint and patience.