Brake Master Cylinder problems of the 2014 Infiniti Q50

My 2014 Infiniti Q50 sport has an unresolved reoccurring problem were after aprox. 20 miles of driving the brake pedal will sink substantially towards the floor after braking. As the car will still stop after the brakes are applied the braking distance is longer than normal and the feeling of security is greatly reduced. To date the car has been back to the dealership 3 times for this issue. The first time a full brake job was performed at my expense. Within a couple months of the brake job the problem came back. The car was returned to the dealer in July 2016 for further evaluation. After 2 days of short trial road testing the car was returned to me with the explanation that the dealership "could not replicate the issue" one week after that the problem occurred again and I limped the car back into the dealership as the problem was occurring. They semi acknowledged the issue and proceeded to order a new master cylinder that took 1 month to come in and get installed. I received the car back on 8/11/2016 and immediately noticed the pedal was still lower than normal but operational. After driving for about 50 miles on open highway the problem reoccurred again. Once again I took the car back to the dealership where it lies now with no resolve. At this point I am afraid to drive the car because of this issue and the many past recall issues involving air bags and the electronic steering system. At this point I am looking at turning the car in with 10 remaining lease months left at an expense of $7,000. 00. I am not willing to put my life or the lives of my loved ones in jeopardy.