2016 Mazda 3 i sport
 

I just recently changed the rear brake pads and rotors on my 2016 Mazda 3 i sport. I have the special tool to compress the caliper and everything. Now I am a professional diesel mechanic so I am pretty proficient in mechanical work. The pedal is sinking all the way to the floor when I am driving it. Now if I pump it 2-3 times it will harden up, but as soon as I am going again, soft and to the floor barely stopping the car. I have checked the fluid and the calipers. No leaks anywhere. I really don’t want it to be the master cylinder cause it only has 80,000 miles on it and never had an issue ever. Any help is appreciated