Topic: What Did I Do Wrong?

[Vintageguy]

1978 750F Super Sport. Rear brake problems common to this model, as I understand it. Mine were sticking to the point that it could be felt riding. Like having, well, the brakes on. Took off caliper and pushed the piston out just a bit, Cleaned it completely with chrome cleaner, and dug out a good deal of crud under the outer seal at the bottom. Pushed the piston back in with a C clamp and rerinstalled the caliper.  Now have no brake pressure. Full of fluid. Did I destroy a seal under the piston or elsewhere in the caliper? I did feel a good deal of pressure pushing the piston back in. Thank you for any/all responses.   

[denward17]

Could you or did you bleed it after re-installing the piston?

[bryanj]

Pushing piston out "a bit" wont work, they need to come all the way out, remove seal, scrupulously clean seal groove, fit new seals and if any pitting new pistons.
Master also needs cleaning out and both large and small hole cleaning properly

[69cb750]

Pushing piston out "a bit" wont work, they need to come all the way out, remove seal, scrupulously clean seal groove, fit new seals and if any pitting new pistons.

Correct, piston, seal, groove need to be spotless clean !

[willbird]

Pushing piston out "a bit" wont work, they need to come all the way out, remove seal, scrupulously clean seal groove, fit new seals and if any pitting new pistons.

Correct, piston, seal, groove need to be spotless clean !

And we all know this but some of us remember learning it :-). The groove that the square O ring fits into is wider than the O ring, and it must be completely clean. The O ring moves into that extra space when pressure is released and this is what retracts the piston (if it is similar to the front caliper). My front caliper was dragging and it was because that groove was full of "stuff", it often has to be mechanically removed, I used a little wad of scotchbrite held in needle nose pliers.

[Vintageguy]

Needed to bleed out line. Took air, but did not think so at the time. Think I'll just get a new kit and do the whole thing properly, as noted. Thank you for the help. This forum is very helpful, and remarkably judgement free.

[newday777]

Was the master cylinder empty before bleeding?
Did you use the banjo bolt at the master cylinder as your 1st bleed point before bleeding the caliper?

[70CB750]

Needed to bleed out line. Took air, but did not think so at the time. Think I'll just get a new kit and do the whole thing properly, as noted. Thank you for the help. This forum is very helpful, and remarkably judgement free.

Fragment of a piston ring is great tool for cleaning the inside grove of the caliper cylinder. 

[bryanj]

Easier with a dental pick or one from the sets you can buy just not a wire brush on a dremel as all that does is polish the crud

[Vintageguy]

I think I know what at least part of the problem is; Whoever rebuilt that caliper last, failed to include the piston seal. Minor point, but a really big deal.