I assume that the one you are now trying to get apart is junk? If so, insert your new key into the old lock. You'll notice that some of the brass tumblers are up, and some are down. The ones that are up are the ones that keep the lock from functioning as it should. If it is scrap, file down the high tumblers to flush and your new key will work in the old latch. Do the disassembly process as outlined before, pitch the old lock, install the new one in reverse order as disassembly and your done.
