Wilwood Spot Brake Installation Question

I have had these spot brakes for a few years now and only getting round to installing them now. I have scoured the www and cant seem to find any installation instructions for the caliper. Has anyone had experience with these?
 
The part Im struggling with is the loose 'piston', brake pad backing plate and brake pad itself on one side of the caliper. I can see the pad is actuated by the cable pulling the lever which pushes the little 'piston' against the backing plate. Are these bits just designed to just 'float' and push off the brake disc, as I dont see how the brake pad wont fall out once its worn?