Brake Master Cylinder problems of the 2014 Mazda CX9

First brakes started failing were soft had to pump them to stop in time as to not hit another vehicle. I took my car to my mechanic. He said we needed to replace the master cylinder. Did this now brakes make a whomp whomp sound and push back some. Was told to drive a few days and see if it gets better. Did this and hubby was driving they were soft again. Then I took car today it was fine until I got home to pick up kids to take them out left car running came in got my kids went back out to car. At this time brakes are now hard and you can't push them down. I put car I'm reverse and car didn't want to move almost like brakes are on safety bake is not on. I took around block to see if it would get better. It didn't in fact I barely made it up a small hill even with my foot to the floor with gas peddle. Got back to my house opened door and you could smell the brakes like they were burnt. Will be taking car back to machanic tomorrow. I know there has been problems with other years mine is a 2014 I don't think they have fixed or solved them. My car only has 71,000 miles on it right now. Note when I say car I am meaning SUV. My car when it first happened mushy brakes was in motion on road not freeway thank goodness. The second problem was not in motion to begin with then was in motion.

Vehicle has 57000 miles had to put intwo master cylinders in two years. Auto was in motion and complete loss of brakes.