I tend to drive pretty conservative so the extensive, full travel bleeding most likely pushed the rubbers to their final end.
I'm old enough that we used to rebuild Master Cylinders as the default, and only swapped them out when the original
unit was beyond repair.
Anyway, I just wanted to share that your last sentence is spot on. Back in the day we would have a vehicle with a
firm pedal, but noisy wear squealers brought the customer in. We would do a careful brake job on all 4 corners, bleed the brakes,
and no matter how much fluid was used after the last air bubbles appeared we'd still end up with a mooshy pedal? And
when you removed & disassembled the master you would see that the bore looked perfect where all the normal driving
pedal travel occurred, but in the untraveled area the bore would be a rough, rusty horror show. It was easy to see
how the rusted walls would shred the rubber seals (once pushed into this area) with a cheese-grater like efficiency, almost instantly.
Essentially the old master cylinder seals were fine right up to the point where we were giving the brake pedal
full strokes
during the bleeding process. Some master cylinder bores would hone back to new, and with a rebuild kit it would be
bench bled, reinstalled, brakes rebled, and sure enough a firm pedal would magically reappear.
On the other hand, some of the really rusty/pitted ones were just too far gone and would require a new replacement.
Since the rebuild kits were affordable and I was a $4/hr labor cost (45+ years ago :0) ...a new MC back then usually meant
much extra sadness for the customer when it came time to pay the bill. (I do remember the boss would keep the damaged master
for the customer to see that we weren't artificially inflating the bill and also tell them that renewing the blake fluid according
to the PM schedule in the owner's manual would help prevent this from happening down the road.)
I *do* remember that the owner's rule was that no vehicle left the shop with anything but a firm pedal. This was one
of the better life lessons I learned from the time I spent apprenticing at a 1 1/2 man shop back in the day. :0)
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The fact that the master cylinder sees a much longer pedal travel than normal during classic bleeding procedures is a
good thing to keep in mind. Once you see your first master cylinder bore after it has been in service for awhile with
unchanged brake fluid is a genuine eye opener. Makes perfect sense.