2000 Hyundai Accent power booster failure

Have you checked for vacuum leaks to the booster itself?

I have check for leaks to the unit, none found.
I will replace brake booster just to be on the safe side.

Before replacing it, to a vacuum test on it to make sure it is bad. THey are not cheap and are a pain to replace. You don't want to do it if you don't need to.