Since rom manager uses flash_image to re-flash recovery (see /data/data/com.koushikdutta.rommanager/files/ )
Something sounds like user error here, since if flash_image really couldn't flash anything you could not reinstall your old recovery.
(I understand its not built into the rom by default thus you need a static build of your own.. I know sometimes flash_image fails.. but erase_image and retrying usually corrects that after the phone boots more.. I know these low level flash applications must be run as root. I know cwm and ra recoveries have flash_image built in thus you could do this from recovery via adb. I know if you are using cmtd there may be some oddness with the size of the cache partitions in/out of recovery.)
This is even without properly asking is before touching recovery asking yourself: do you want to touch recovery? Since recovery is what you use to recover a bad system.
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I followed the guide that I mentioned you, following step by step (I used Better Terminal Emulator for copying it to system and apply the 777 permisions)
Still the only thing that it worked is the ROM Manager, I'm curious thought, if it uses the same flash_image I don't see why it would fail my method...