Yes - it should. When you refer to the auto-brightness button, are you referring to the setting in the iPad's 'Settings' app? Also, be careful you do not have the auto-brightness sensor covered. It's virtually invisible, but it's towards the top of the iPad case, on the front, in the centre of the screen and appears as a very slight lightening of the black border around the iPad screen. Some Members have even thought it was a fault in the colouring of the screen surround.
Tim