Sounds like you need a Honda shop manual if you don't already have one. I would suggest the Honda version over the after market brands which contain errors & omissions. Good investment.
www.helminc.com or 1-888-292-5393. Hopefully this info is current. America Honda provided this info to me 4/01. You have an old bike and old bikes tend to have issues. I have 3 different manuals for my 75 750F and they are pretty grease stained.
Please clarify which brake. The front may be as simple as turning the adjustment screw located by the fork leg. Hopefully the expensive pistons don't have excessive corrosion. Make sure you use a good solvent like acetone on the inner surfaces (eats paint-be careful) of the caliper and the piston to dissolve any residue. I use a felt wheel (non-abrasive) wet with acetone on my Dremel to polish/clean the surfaces and Q-Tips or the bristle wheel & Dremel as Super Pasty said for the seal groove. As the guys have said get the seal groove cleaned well and use a new seal. When you reassemble be sure to "use silicone sealing grease (temp range of -67° to +392°F)" as the shop manual says which will lubricate and SEAL out and moisture or dirt."Do not use molybdenum grease aka brake grease" per the manual as this won't handle the heat and will not last. Apply this to the surface areas of the calipers where the outer edges of the pads slide. Do not get it on the faces of the pads. Part of the seal's job it is designed for is to pull the piston back into the caliper. Don't know that I could provide a decent explanation of this though.
I'm about to bring up a couple things that have been highly debated here. One is what silicone compound to use. Some of the guys only want to use a particular brand/type of grease and say that's all you should use. I don't remember which it is but it's difficult at best to find. It's a really high grade and will do fine. I use NAPA Sil-Glyde lubricating compound. One of it's listed uses is brake parts. "Stays put. Water repellent. Won't run or melt. Effective from -20° to + 400°F. Harmless to rubber." The second debate concerns which brake fluid to use. THE BOOK says DOT3 which is fine. When THE BOOK was written there was no DOT4 or DOT5. When, not if, your seal leaks DOT3 is going to eat up your paint. I had my fork legs, caliper, caliper bracket, and center of my disc powder coated due to paint damage. When I put it all back together I went with DOT5 (My Harley came from the factory with DOT5) and it's working great. No problems. I made sure not to mix DOT5 with DOT3. I flushed out my master cylinder and brake line by pumping 2 or 3 fillings of DOT5 through the system until I was satisfied with my results. I actually have DOT5 in the front & DOT3 in the rear. Next time the rear comes apart I will go with DOT5. Also if you want a hassle free job of bleeding the brakes go to the brake FAQ and look up Speed Bleeders. You'll be glad you did. Just a couple things to think about.
