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Reviving an old thread that I hope the OP is subscribed to. What were the symptoms that prompted you to replace the master cylinder?

I have a 2005 with 85k miles that has been to over 40 track days and tons of autocrosses. I have been having an inconsistent pedal for a very long time. The best way to describe it is during mountain drives I get into the brakes and have a soft pedal, release and immediately reapply and the pedal is firm as I expect. It's not during corners, straights, or anything repeatable, it's just inconsistent. What is consistent is that the first application is soft, but the immediate second application is firm.

I had a shop take a look and they determined it was operating as normal... not much help there. They recommended I replace the rotors and pads as they were well worn, so I did. No change. During that process I noticed cracks in the dust boot on one of my calipers, so I rebuilt all four of them. All four had cracks in the boots, but were not leaking, though I do think the inner seals were a bit worn. Now I have four rebuilt calipers and still the inconsistent pedal remains.

I know the system is air free... not a rookie at this bleeding thing as I flush the system before every track event. I'm open to suggestions, but I'm leaning towards a new master cylinder. I don't know much about them or why they would fail. Inner seals maybe?

Thanks in advance!
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