Having a similar condition with my 2011 MKX. After doing some investigating, no external leaks, brake petal solid when it works, (no internal leaks in M/C), went to the dealer and talked to a mechanic and he informed me there is a dump valve on each brake caliper, and one could be stuck open ever once in awhile. My brakes work fine, except for the fact that they go to the floor at times. Not often, but when it does it will do it 2-3 times in a row, then all is good for a while (months B-4 it happens again) Plan on flushing out my brake lines this weekend, then wait and see if it happens again. Problem is, I can't replicate the problem, it happens when it happens.