Topic: '77 CB750F Rear Master question

[Stoli]

The rear disc brake on my son's 77F was sticking, even after we had done several transfusions last season. We had suspected the small return hole in the reservoir was getting clogged with some contaminates in the fluid. Anyways, we decided it would be a good idea to do a complete rebuild of the master cylinder so that's what we did today. In the process, we found that the "stop plate" was severely cupped. The stop plate is the washer that sits just under the circlip. Image below (not of the bike in question) shows the washer on assembly B.



Is it supposed to be cupped? If not, has anyone seen that and how did it get that way?
 Most of the pics that I have found make it look like a flat washer.

[andrewk]

Probably excess force on the return stroke...  People sometimes will pull up on a sticky brake in an ineffective attempt to get it to release.   If the return stop bolt is bent or isn't adjusted properly, as many are, you could easily cup the flat washer.

[Stoli]

Makes sense. I didn't consider that because I don't think the pedal was sticking down on my son - pedal travel was normal but the caliper was not releasing. Maybe the extra travel on the piston due to the cupped washer was the reason? Not sure.

Flattened out the washer and reassembled so hopefully it will work now.