It's not a mechanical stop thing. The bore of the M/C will over time develop a "ridge" on it at the point where its piston normally stops during braking. Bleeding will cause the person pumping the brakes to press beyond that ridge and the seals on the piston can sometimes get damaged by traveling over that "rough" spot, leading to fluid bypassing.Man that damage thing is a new one to me, I've been bleeding brakes since forever and I do change out the fluid every few years on all my cars. The person in the car is always my wife or one of my daughters and I wouldn't say they have been all that careful nor gentle with "down" when told to do so.
Wouldn't there be a mechanical stop on the pedal assembly to prevent damage to the master cylinder?