Spoiler: The Deathly Hallows?

All through book 6, Dumbledore could have had all 3 hallows by simply borrowing Harry's cloak, as he already had both the wand and the stone. And the stone still worked as a resurection stone, since Harry used it at the end of book 7. Why didn't Dumbledore conquer death - or at least put his own death off long enough to help Harry kill Voldermort?