I have seen weld failure/crack where the torque convertor drive hub is welded to the torque convertor shell. A hairline crack in the weld will exactly mimic a defective or damaged pump seal.
We were actually able to see the crack under a strong light with a magnifying glass, but we did not think of looking for one when we had it down the first time, so it was a "do over" job when we looked for a crack.
After finding the crack we improvised an air hose adapter to apply shop compressed air to the torque convertor through the seal hub. With the torque convertor lying flat on a table, seal hub facing up, we filled the center area around the seal hub with a bit of soapy water. Applying just a small amount of compressed air to the adapter hose with a hand nozzle caused some serious bubbling around the cracked weld.
That torque convertor was in a Toyota 3FGCU25 (if I remember correctly).