Howdy, new guy on this forum. My son has a 2016 base v6 mustang 6-speed manual with 106k miles on it. He had mentioned to me that he thought it was time to replace the front pads and that the brake pedal had been sinking slowly when stopped at an intersection. I confirmed the pedal behavior. When you first press the pedal, it feels fine, but when held, it slowly sinks. We replaced the front brakes last week, new pads and turned rotors, no leaks on front calipers. The rear brakes were in excellent shape, so we left them alone after removing and inspecting the calipers for leaks. After the brakes were completed, my son took the car to the dealership and had the brake fluid flushed. He reports that the pedal still sinks as noted before. At this point I am guessing that the brake master cylinder is bad and we should replace it. I am fine with this, but I havnt been able to find any steps or youtube videos on ~2016-ish mustangs for how to properly replace the brake master cylinder. I am also wondering if the issue could be with the brake booster or perhaps a vacuum leak? Does anyone have access to a pdf that shows the process? I did search this and multiple forums for instructions and/or videos.