Motorcycle Repair: brake sticking, piston seal, dish soap

Question
The back brake keeps sticking on my '77 cb750f. I've replaced the piston, seal, and rebuilt the master cylinder. The inner wall of the caliper looks fine. I've also soaked the lines in dish soap to clean it.

-Jason

Answer
Jason, I think you'll find the o-ring groove still has junk in it making the piston bind up.  The brake fluid makes a pretty tough varnish like coating in that groove.  I usually take a dulled bent nail or something like it to scratch out the stuff.  Just be careful not to damage the groove by scraping too hard.  The varnish is almost clear so it's sometimes hard to see.  If the piston doesn't slide into the bore pretty easily with the o-ring in place, it's still dirty. Just be sure you put a little brake fluid in the bore to lubricate things when you try this test or it will bind up.

Regards
Rich