Is the only way to receive the broken condition to take damage? If so, then the definition of immunity would make "Unbreakable" immune to damage vs object.
"Immunity (Ex or Su)
A creature with immunities takes no damage from listed sources. Immunities can also apply to afflictions, conditions, spells (based on school, level, or save type), and other effects. A creature that is immune does not suffer from these effects, or any secondary effects that are triggered due to an immune effect
Format: Immune acid, fire, paralysis; Location: Defensive Abilities."
Yes you have to reduce the object ot 75% of its hitpoints for it to get the broken condition.
Forgive me for possibly resurrecting a dead post, but let me posit this querum:
For anyone who plays 4e, there is a condition called "bloodied" which occurs whenever a character (PC or NPC) reaches 50% of their total hit points. Certain abilities only work when bloodied, some only work until bloodied, some provide additional effects when used on a bloodied opponent, so on and so forth. There's at least one ability that allows a character to act as bloodied even when they aren't actually bloodied (so as to benefit from other effects that are only usable when bloodied, with the drawback of being vulnerable to effects that only affect bloodied creatures).
If there were a magic item, for example, that said, "A character wearing this item is immune to the bloodied condition", it would provide obvious benefits (such as being immune to something that deals extra damage to bloodied creatures) while at the same time providing obvious drawbacks (such as the inability to use certain powers that only function while bloodied).
But would such an item make a creature immortal, since they cannot achieve the bloodied condition (which, like I said, occurs at 50% hp)?
Also, let's call this item "the Amulet of Guarded Blood". Would your answer to the previous question be different the item was called "the Amulet of Immortality"?
I think this a perfect parallel, but might show things from a slightly different angle.