Replaced the leaking (less than a year old, almost no miles) left rear caliper(2014 Mustang GT) today. I had noticed that the front reservoir of the master cylinder had dropped about halfway. Couldn't see any signs of fluid in the driveway on wheels, so I had to get underneath and look around. I found evidence of dampness on the left rear lower slider boot, so it was apparently dripping down out of the main piston boot.
No warranty, so I bought a reman off RockAuto
Installed, and bled
Also aftermarket pulse damper arrived today, so installed that as well definitely makes the fuel rail quieter at idle. Never seen a factory one that required a vacuum reference, but this one is intended to have it.