I would try bleeding it again and make sure it has plenty of pressure pushing out the air and fluid out the bleeders,,if that doesn't solve it you may have a bad booster ,,if your not getting pressure out the bleeders when you bleed that may be the new master cylinder,,but if has plenty of pressure when bleeding it must be the booster,,but i have never seen one with the symptoms you describe,,so it is strange but someone did state here they have run into a booster doing what you describe ,, i know it sucks to change the booster with the seats installed and everything else in the way,,hope you get it solved