It's a bit weird sometimes, i agree.
One tip: it takes up to a second or two for the catapult to actually begin moving, so be patient when trying to toggle the break.
Also, it seems that certain terrain does not allow it to move forward, thus making it either do nothing (when trying to move forward) and then move backwards.
If all is working correctly you switch between these states: 1) moving forward 2) stop 3) moving backward 4) stop 5) moving forward etc etc etc