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2004~2008 Brakes soft after replacement and bleeding

MazMetalhead

I like manuals, Mazdas, and metal music.
:
New England
:
3 MT, 5 AT, 6 MT
Hi everyone, looking for thoughts. I replaced rotors and pads on my '05 3, and while the pedal is very firm with the engine off, but completely softens with the engine running. Like, you have to use a tremendous amount of force to make the car stop.

I bled the brakes after changing them using the correct order (passenger rear, driver front, driver rear, passenger front) and I used this one-man kit with the check valve so I'm pretty confident I didn't introduce any air. Do you think I should bleed again or do you think something else might be wrong?
Screenshot_20200314-170038.png
 
:
2014 Mazda3 S GT auto, 2008 MX5 6-speed
Was the pedal firm before you replaced the rotors and pads? How many miles are on your odometer? I ask because bleeding the brake fluid by pushing the pedal to the floor can, especially if the master cylinder has a very high number of strokes under its belt (so to speak) may result in a failed seal in the master cylinder.
 

MazMetalhead

I like manuals, Mazdas, and metal music.
:
New England
:
3 MT, 5 AT, 6 MT
Was the pedal firm before you replaced the rotors and pads? How many miles are on your odometer? I ask because bleeding the brake fluid by pushing the pedal to the floor can, especially if the master cylinder has a very high number of strokes under its belt (so to speak) may result in a failed seal in the master cylinder.
The car has 203k miles and it was neglected for the past couple years at least (not by me, I bought it at the beginning of the year). I've definitely thought of the master cylinder. The brakes were squishy when I bought it but I attributed that to all the brakes practically being metal on metal. They're a little firmer now with all the work done. But it's just weird to me that they go soft when the engine turns on.
 

MazMetalhead

I like manuals, Mazdas, and metal music.
:
New England
:
3 MT, 5 AT, 6 MT
With that many miles on the MC, it should be a prime culprit, IMO.
Most likely, but the pedal doesn't sink to the floor or at all. And the pedal only softens when the car runs... It's very strange. And the car does stop fine, it's like the first half of the pedal travel is wrong and it firms up halfway down.
 

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