In general, a master cylinder failure will involve one of the two (68 and later) cup seals.
For a high mileage car moisture collects in the bottom of the master cylinder bore and creates rust pits. The front inner cup seal rides over the pits and the rubber get abraided. Eventually a groove is worn into the cup seal and its sealing function fails and pedal goes almost to the floor.
The rear cup seal is at the end of the cylinder bore near the pushrod (at the mounting flange). Every time the pedal is depressed the rear of the bore wall is exposed to the atmosphere. Eventually rust develops here. Every time the brakes are released the rear cup seal rests in the rusted area. The cup seal erodes and fails. This is the most common failure of the master cylinder. On the 911's in the pre failure mode you will notice paint corroding on the booster under the attachment flange of the master cylinder. For the 928's and 44's you will notice fluid leaking down the firewall underneath the carpeting at the master cylinder mounting flange. Another symptom of rear cup seal failure is that in manual bleeding the rear brakes will not completely bleed out. There will be little air bubbles that will not go away. However pressure bleeding will remove all the air bubbles because the brake cylinder is not actuated. After some miles of usage the bubbles will return.
Yes, the master cylinder can be rebuilt providing the bore is not too big in diameter after honing. For cylinder bores 19mm or larger the bore tolerance is at least .004" (a little bigger for larger bores--listed in the instructions in kit). High mileage (100K or more) cars usually will exceed the limit without honing. Honing out rust pits is guaranteed to exceed this spec.
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