ok i just had the same problem on saturday with my 74cb550 front caliper. compressed air everything nothing worked. the best thing to do is get a grease gun that will fit onto your bleeder valve and find a bolt with the correct size and thread where the brake line goes and plug it, a little grease may come out the threads but thats ok. as you pump the grease in make sure you push down on the end that is on the bleeder valve and you will see the pistoon come out. go to
www.oldmanhonda.com and there is a great description on how to clean it up and out and reuse the old oring. i used 2 small screw drivers that came with stihl chainsaws to clean out the groove. i grinded one down to a small flat thin point to get the oring out without messing it up. then i heated and bent the second screwdriver and grinded the sides down a bit to fit in the groove perfectly and scraped out all the junk in the groove. carb cleaner will help alot in this proces. good luck let me know if this helped! it worked great for me!
