2005 Navigator

Working on a 2005 Navigator. Came in with the brake pedal going to the floor. I recommended a master cylinder. However, afterwards it still seems to have an easy pedal sometimes. I've replaced the master cylinder twice, bled it 3 times, checked and lubricated the slides and there is still quite a bit of pad life on all 4 wheels. Any help would be great. Also, there is a buzzing sound under the hood when I start the car up. Think that's just air ride because there is no ABS light on.