Failed internal seals on the master piston, or some corrosion in the bore (Brake fluid is hygroscopic after all) (?) That would account for the dropped pedal overnight as fluid slowly seeps back to the resevoir under the weight of the pedal. That would also explain why on hot days (metal cylinder expands but rubber doesn't - well not as much) there is a change in pedal travel.
Give it a set of new seals (Both master and slave) a hone and a clean. Clutch cylinders don't have to be as perfect as brake components, so maybe just a clean up with emery paper might suffice to smooth out any corroded areas or ridges.


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