My 2008 trailer was doing the exact same thing. My dealer said it was a comon problem and replaced a dampener shock in the actuator. This was back in 2010, so I'm not sure if they can get it any more, but it solved the problem. The trailer brakes still work great to this day.
Check out this link. Scroll down to the parts diagram. Look a t #6. The Reliable part # is SA-001