I don't have the 9A plans but if the 'brake' is still a pair of 2x6 with a hinge, you probably made it too well. You want there to be a gap at the hinge, about the same as the dowel. Otherwise you can never overbend the skin enough to keep it from springing back. You can probably make a little more progress by putting the skin back in the brake but not all the way in; say, 1.5". You'll gain a little more angle that way, maybe enough.
As far as backriveting the tab, I was able to backrivet the inner flange on my 6A tab by just curling up the skin. Start at the narrow end and work toward the other. You won't 'unbend' the skin enough to make a difference if you are gentle. The other flange can simply be squeezed. It's easier to squeeze the hinge and flange with the hinge disassembled but you may find the hinge shifts enough to get suddenly tight. I solved the problem by riveting the hinge to the elevator first, while assembled. Then I disassembled the hinge and assembled the tab side with a spare piece of hinge, which kept the hinge straight while I riveted it to the tab. Another tip: fold the hinge so that it is 90 degrees, not flat, while you rivet it. This keeps the hinge aligned in two axis and prevents you from putting a bow in it. This is also a useful technique while drilling hinge - unless we're talking the cowling, where you want the bow.
