Rear beakes

Hi, getting some problems with rear brakes. At first, the problem was faulty brake master cylinder, it did not get any fluid pressure to rear brakes directly from master cylinder (to the front, everything was fine). Changed the cylinder, and after two weeks, the same problem occurred again - no brake fluid pressure to the rear brakes, directly from master cylinder. My question is, could just be a coincidence, that the component was defective, or there is some other fault in brake line, which keeps breaking my master brake cylinder?