Rear disk brake seizing due to issues with master cylinder


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Hi, I'm having issues with my rear disk brake seizing on when the system gets hot.
It's not the Calliper as I've already re-done the seals. It's not the fluid as I've replaced it a couple of times, most recently when I re-did the calliper piston seal.
I've narrowed the problem down to the Master cylinder. I think fluid is coming from the reservoir but not returning to the reservoir properly, almost like the master cylinder piston is acting like a non-return valve. that seams unlikely though, it would be hard for it to get stuck in a position like that, and when you blead off the systems bit or let it cool down it goes back to working ok.
Anyone seen this happen or have any ideas?
I had the same problem with mine after I first resurrected it. It turned out to be the adjustment of the brake pushrod. I went back to the instructions in the factory manual and followed them exactly and VERY carefully and that solved the problem.
There are two ports between the bore and the reservoir the main one is fairly large. The second called the bleed off port is the likely culprit, it has a very small hole, easily plugged. When it DOES get plugged; exactly what you describe, RE brake caliper binding, occurs. What Mike G describes is the same problem but the piston blocks the port if the rod is improperly adjusted.
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Happened to me. My rear brake locked up just a hundred feet from my driveway. I let it cool down and was able to push it home. Tearing down and rebuilding the MC solved the problem.
How did you get the MC apart? I have the circling out but then what?