1978 KZ 1000 Rear Master Cylinder

78 Chain drive single rear disc. RH. My rear master cylinder builds up pressure and doesn't release. I will bleed the pressure at the caliper and then it seems to work for a while, a couple of hours of riding stop and go, and then pressure builds up again. Has anyone seen this? Will a master cylinder rebuild kit fix this? Is it rust, debris? Not able to find an OEM replacement. Would prefer to avoid modifications to accommodate a different make or model master.

sounds like your return port is clogged. take apart the master cylinder and look at the VERY tiny hole. make sure its super clean. since you're in there i would take apart and inspect and clean the entire MC and caliper.

+1 on that. Pull it apart and give it all a good clean.If youre careful you might get away with reusing the rubber bits and not need a rebuild kit.
If not,a rebuild kit isnt expensive and worth fitting for peace of mind if nothing else.
Good luck