It's either metric or very crappily made SAE hex. This is what we get for outsourcing everything and one more reason why I always rebuild original factory wheel cylinders.
Sorry guys, I wouldn't advise using a ign. wrench on a brake bleeder. Those wrenches are stamped steel and don't have much strength. They would spread easily and round off the bleeder. They are designed to loosen machine screws. You would be better off using a adjustable wrench.