Pat, this is a little late, but if you think about what disk brakes do, they're pretty simple.
They just squeeze 2 pads against the rotors, whilst the rotors are turning and the calipers are held still. That differential motion has friction and is what slows down the rotor and thus the trailer.
So to squeeze the pads you need a hydraulic cylinder to push against one pad and since the other end of the cylinder is forcing the cavity the other way, that force can be used to force the other pad also against the other side of the rotor. Both forces have to be equal, Newtons Second law. So that's how you get the squeezing action.
Since the pads wear, you need a sliding pin arraignment to take up the wear, and that's where those 2 allen head pins come in. They let the caliper body slide one way and the piston press the other. And when you replace pads, you need to squeeze the piston back in, as you did.
Simple when you stop to think what those calipers have to do.
Boris