OK, just got done tearing mine all apart this time, and sure enough, there was evidence of slight leakage inside the boot of the master cylinder, even though it's only ten months old. There wasn't enough to leak past the boot and show anywhere outside of the cylinder, and not enough to visibly lower the fluid level, but clearly enough to get air inside.

I took the MC apart, and cleaned and rinsed it inside with clean brake fluid. You can see a little wear on the cylinder walls already. I also changed out the pushrod for the longer of the two, which more closely matches what was in there to begin with. I did that repair on the road, 600 miles from home and the internet, and was having a hard time fitting the long pushrod for some reason, so I used the shorter one.

I bled it up again, with the caliper, and now it's a working clutch again, although it releases pretty close to the floor. Clearly, I'm going to need another MC soon. And then I'll get to bleed it all over again.... :-(

Is there a place I can get just a rebuild kit for this MC? I can certainly hone it out and put new rubber in it, and I would rather do that than cough up for a whole new MC again.

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1976 Mercedes 240D, unknown mileage
1992 Dodge/Cummins 4WD, 284k