You need to check the internal parts for wear with a feeler gauge, and look for score marks on the bottom plate where the gears rotate. Also dismantle the relief valve and check it for freedom of movement and proper spring pressure. I've seen "good-looking" pumps that were severely worn or scored inside, and that allows oil to recirculate inside the pump. The result is reduced volume to the engine, and/or low oil pressure. A pump would have to measure out close to brand new before I'd ever reuse one on a rebuild. A new pump is cheap insurance on an expensive overhaul job!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!