On the SS, you should have the servo arm/saver, and another 90 degree horn on the side brace near the engine. the only adjustable piece is the ball link from the 90 degree horn to the carb.

The idle on the motor is set at the factory. Mine was fine, and yours is likely OK as well. I'm guessing that you have the 18T gear on the motor, becasue with that set-up the motor is further away from the 90 degree horn than the instructions assumed (15T is a bit closer). So, when you set the length of the ball link per the instructions, a 90 degree servo position is not idle. If you set the trim for a decent idle, then it's hard to get enough brake because the servo arm is already about 15 degrees past 90 degrees.

To fix this, remove the ball link piece from the carb and from the 90 degree horn. Unsrew the ball cup (lefty loosey) until you get it to a length that sets the servo arm at 90 degrees. Then set up the brake. This should work, but you have to go through the same thing again any time you change clutch bells.

I solved this whole issue by changing the entire set-up. If I figure out how to post pictures, I'll put them and some instructions on the site.