We have our pistons extended about 1/3 of their 8" stroke. We don't use a latch on ours...instead, we rely on the geometry of the system to keep the ball from launching until the cylinders build air pressure, and then whammo, they move and launch the ball. The cylinders start out almost parallel to the arms.
I think it works ok, we were finalists...but if you want to build a really good one, find team 842's web page, where they show how they built theirs. It's different, and better.
https://sites.google.com/site/falcon...ots/2014-dream