That o-ring turning into confetti is rough, I've had that exact thing happen on a Friday afternoon with a rush job waiting. Idk what brand you're using but if it was a standard cheap rebuild kit it probably used a Buna-N o-ring that just can't handle the heat and the constant vibration. You might want to grab a urethane or a Viton o-ring for the rebuild, they hold up way better under high cycle use. I mean it costs a couple bucks more but you won't be tearing it apart again next week. Also check the cylinder walls for scoring while you have it open, sometimes that shredded rubber hides a burr that'll kill the next seal too.