Hey all, just talked to my buddy whos a ford warranty repair regional mgr.
He said get off the clutch pedal when you are driving! Its causing heat at the tob, blowing seals on the CMC and Slave, and in general making this issue a thing.
Funny thing is, I have 175k on mine and it doesnt have this issue. His 14 mustang is also ok and we both beat the cars hard....but properly.
I got to this thread while looking for a 60k mileage range FiST, and one had his go to the floor on a test drive and he reduced the cost after. The other said he just did a mast cylinder job on his @61k ...which made me go googlin on WTF is going on here.....and asking my buddy.
By this time, you are too late and now ur in "Clutch/tob/slave/master/line flush" territory, so try not to use the clutch pedal as a footrest! He also said listen for a loud TOB as the indication you have wear that is going to be a problem.
*Might be related to the "bad batch of tob" discussed above, im not saying this couldnt be a thing as well!*