We fixed the binding issue. I did have the piston in upside down, but I decided to replace it anyhow. I figure I might as well since there is more pitting on there than I'd like. I was looking at a build thread earlier for a 74 CB550, and I noticed that the calipers are remarkably similar looking. When I did research on sites that sell OEM parts, they actually accept the same pads, bleeder screw, and hard pipe, but they have different piston seals and possibly pistons altogether as they are still available for the 550 but not the 750.
Before I stumbled across that build thread this morning, I purchased a new piston on eBay for what I thought was the 77-78 CB750K caliper that I have. How easily should it slide in with the caliper seal in? I purchased a new 77-78 piston seal, and althought it seems to fit pretty snug, the old piston would not press in fully just by hand. I used a large socket and mallet to gently tap it in. I just want to make sure I have all the correct parts!
Pic of my caliper for reference:
