[chrismorse]

When I worked at a dealership, as a service writer, I was often asked to do the up-down with the brake pedal, while the real mechanic bled the calipers/wheel cylinders.
On older cars, (10+ years), we began to occasionally see master cylinder failure after manual, (foot), bleeding.
Power bleeding prevented this, (vacuum bleeding works well too).
My thoughts were that moisture, rust and small bits of deteriorated brake components, like rubber hoses would collect in the master cylinder. Each brake application would sweep the bore clean -to a mid point in the bore. Over time, this debris, solidifies and it is easy for a full brake pedal stroke to push the seal over the trash, occasionally cutting the master cylinder piston seal.
With newer master cylinders or annually flushed systems cutting a seal would be highly unlikely, but 20, 30- 40 year old cars, especially infrequently driven or flushed may be at risk.
Pressure, vacuum or gravity bleeding avoids this risk.
chris