Have drip from spout in tub/ shower.I have had plumber out twice. He changed valve and valve seats both times. After 6 weeks drip is back. I relaced washer with beveled washer still drips. 40 year old house with price phister valves

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he changed valve and valve seats both times. After 6 weeks drip is back.

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I'm not accusing the plumber of blowing smoke up your skirt when he said he changed the seats. (Did he or leave the old seats so you could see? )
But if the seat were rough or nicked it would take about 6 weeks to tear up the new washer. That would would be my first guess. Good luck, Tom