The handbrake is a seperate system from the hydraulic system. It works with the handle pulling a cable that pulls a lever on the leading brake shoe.

My thought was low fluid but you said that was full. I was thinking that when you hit the brakes hard the one time that the fluid level went down in the MC far enough to trigger the light. As the brakes released the fluid came back up. Also I thought that the fluid was low and when the brakes get warmed up, the fluid expands enough to raise the level (but I don't know if that can happen like metal expanding). How full is full?