I've got no problem with Hondaman's posting and explanation of piston retracting.
If your caliper is not retracting properly, there can be other causes.
1. The rubber o ring may not be new stock and 30 years old. Rubber can get hard with age and looses elasticity.
2. Your backing pad may not be adjusted for the .006 free clearance. Which is the approximate distance the rubber oring should retract the caliper piston. The square section rubber, parallelograms when the piston moves. Releasing hydraulic pressure allows the rubber to return to square, and retracts the piston thus. This assumes there is no binding of the piston in the caliper bore, resistance of the brake puck to retract in the bore, or residual hydraulic pressure, as well. There is also a spring on the caliper arm to aid the free positioning of the caliper pads to the rotor.
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5. Speaking of which.... Is your brake lever the stock one that allows the piston in the master to retract fully?
My brake caliper was not letting go of the rotor. Everyone pitched in, and I got the thing fixed. But it was a hard fix for me. I assumed my problem was based in changing from DOT 3 to DOT 5. But like a lot of technical issues (I'm finding) there was more than one cause, and the causes were inter-related in such a way as to make the final, ultimate problem even more complex.
Two Tired's number 5 (above) was key for me. It was astonishing to me how little tolerance for error there is with the lever dimensions, and their relationship with the MC cylinder head.
Number one was important for me to understand too. I cleaned out the o-ring channel carefully (brass wheel on a Dremmel) and then put in the new o-ring. It was interesting for me to see, once doing that, that the outside edge of the o-ring was precisely even with the cylinder wall when I ran my finger inwards. However, the trailing (deeper) inside edge of the o-ring protruded a bit into the cylinder. I don't know if the channel is cut so that it does that, but when the o-ring is just sitting there it looks perfectly symetrical. Then, installed it kinda goes uneven, like I said. You can feel the difference with your finger.
So when I understand how the o-ring grabs the piston like it does, it's easy for me to see how the retraction is accomplished by this minute buckling of the o-ring itself. It's kind of like a mechanical assist to the hydraulic system maybe - I don't know.
Another procedure that I learned was important for me was the backing (inside) pad clearance (number 2, above). I did not realize how important that clearance was. After I used my feeler gauge to set it precisely I was very pleased with how much it helped.
During the whole process (and before I really figured out the exact problems I had) I rebuilt the MC with a Honda kit I got from a friend. Although my old MC may have lasted another while, it was obvious once I had it out, that it wasn't going to last much longer.
Good luck with it.
