Sandpaper followed by scotchbrite on a makeshift mandrel mounted in the cordless drill..
It was badly corroded and had deep pitting.. once I removed the "crust" that had it previously sealed, (but also prevented the piston from fully returning) and saw the pitting, I knew it was probably a lost cause.
The amount of material I had to remove, coupled with reusing the old piston is just too sloppy and won't hold oil. It still builds pressure, and the leak is no worse when applying pressure, but the seal closest to the lever just dumps all the oil from the reservoir.
